Improve PlugStatus

- Display load status
- Load plugin with 'L' key

(This commit also allows not loading a plugin with `'for': []`. It used
to load ftdetect files.)

plug.vim

@@ -80,6 +80,7 @@ let s:TYPE = {
 \   'dict':    type({}),
 \   'funcref': type(function('call'))
 \ }
+let s:loaded = get(s:, 'loaded', {})
 
 function! plug#begin(...)
   if a:0 > 0
@@ -148,9 +149,8 @@ function! plug#end()
   " need to loop through the plugins in reverse
   for name in reverse(copy(g:plugs_order))
     let plug = g:plugs[name]
-    if !has_key(plug, 'on') && !has_key(plug, 'for')
-      let rtp = s:rtp(plug)
-      call s:add_rtp(rtp)
+    if get(s:loaded, plug.dir, 0) || !has_key(plug, 'on') && !has_key(plug, 'for')
+      let rtp = s:add_rtp(plug)
       if reload
         call s:source(rtp, 'plugin/**/*.vim', 'after/plugin/**/*.vim')
       endif
@@ -177,8 +177,11 @@ function! plug#end()
     endif
 
     if has_key(plug, 'for')
-      call s:source(s:rtp(plug), 'ftdetect/**/*.vim', 'after/ftdetect/**/*.vim')
-      for key in s:to_a(plug.for)
+      let types = s:to_a(plug.for)
+      if !empty(types)
+        call s:source(s:rtp(plug), 'ftdetect/**/*.vim', 'after/ftdetect/**/*.vim')
+      endif
+      for key in types
         if !has_key(lod, key)
           let lod[key] = []
         endif
@@ -247,12 +250,15 @@ function! s:esc(path)
   return substitute(a:path, ' ', '\\ ', 'g')
 endfunction
 
-function! s:add_rtp(rtp)
-  execute 'set rtp^='.s:esc(a:rtp)
-  let after = globpath(a:rtp, 'after')
+function! s:add_rtp(plug)
+  let rtp = s:rtp(a:plug)
+  execute 'set rtp^='.s:esc(rtp)
+  let after = globpath(rtp, 'after')
   if isdirectory(after)
     execute 'set rtp+='.s:esc(after)
   endif
+  let s:loaded[a:plug.dir] = 1
+  return rtp
 endfunction
 
 function! s:reorg_rtp()
@@ -287,8 +293,7 @@ function! plug#load(...)
 endfunction
 
 function! s:lod(plug, types)
-  let rtp = s:rtp(a:plug)
-  call s:add_rtp(rtp)
+  let rtp = s:add_rtp(a:plug)
   for dir in a:types
     call s:source(rtp, dir.'/**/*.vim')
   endfor
@@ -339,6 +344,7 @@ function! s:add(repo, ...)
                     \ a:0 == 1 ? s:parse_options(a:1) : s:base_spec)
     let g:plugs[name] = spec
     let g:plugs_order += [name]
+    let s:loaded[spec.dir] = 0
   catch
     return s:err(v:exception)
   endtry
@@ -419,6 +425,7 @@ function! s:syntax()
   syn match plugCommit /^  [0-9a-z]\{7} .*/ contains=plugRelDate,plugSha
   syn match plugSha /\(^  \)\@<=[0-9a-z]\{7}/ contained
   syn match plugRelDate /([^)]*)$/ contained
+  syn match plugNotLoaded /(not loaded)$/
   syn match plugError /^x.*/
   syn keyword Function PlugInstall PlugStatus PlugUpdate PlugClean
   hi def link plug1       Title
@@ -439,6 +446,8 @@ function! s:syntax()
   hi def link plugError   Error
   hi def link plugRelDate Comment
   hi def link plugSha     Identifier
+
+  hi def link plugNotLoaded Comment
 endfunction
 
 function! s:lpad(str, len)
@@ -473,6 +482,7 @@ function! s:prepare()
     call s:assign_name()
   endif
   silent! unmap <buffer> <cr>
+  silent! unmap <buffer> L
   setlocal buftype=nofile bufhidden=wipe nobuflisted noswapfile nowrap cursorline
   setf vim-plug
   call s:syntax()
@@ -1072,6 +1082,7 @@ function! s:status()
   call append(1, '')
 
   let ecnt = 0
+  let unloaded = 0
   let [cnt, total] = [0, len(g:plugs)]
   for [name, spec] in items(g:plugs)
     if has_key(spec, 'uri')
@@ -1089,6 +1100,11 @@ function! s:status()
     endif
     let cnt += 1
     let ecnt += !valid
+    " `s:loaded` entry can be missing if PlugUpgraded
+    if valid && get(s:loaded, spec.dir, -1) == 0
+      let unloaded = 1
+      let msg .= ' (not loaded)'
+    endif
     call s:progress_bar(2, repeat('=', cnt), total)
     call append(3, s:format_message(valid, name, msg))
     normal! 2G
@@ -1096,6 +1112,21 @@ function! s:status()
   endfor
   call setline(1, 'Finished. '.ecnt.' error(s).')
   normal! gg
+  if unloaded
+    echo "Press 'L' on each line to load plugin"
+    nnoremap <silent> <buffer> L :call <SID>status_load(line('.'))<cr>
+    xnoremap <silent> <buffer> L :call <SID>status_load(line('.'))<cr>
+  end
+endfunction
+
+function! s:status_load(lnum)
+  let line = getline(a:lnum)
+  let matches = matchlist(line, '^- \([^:]*\):.*(not loaded)$')
+  if !empty(matches)
+    let name = matches[1]
+    call plug#load(name)
+    call setline(a:lnum, substitute(line, ' (not loaded)$', '', ''))
+  endif
 endfunction
 
 function! s:is_preview_window_open()