Add vimtex support

Indicates:
- whether the file is considered to be main or local
- whether the viewer is opened
- whether the compilation is running
- whether the compilation is continuous

Added:

* `vimtex` existence check
* variables documentation

TODO: readme and a screenshot

Update readme.md

Update doc

Update screenshot url

autoload/airline/extensions/vimtex.vim

@@ -0,0 +1,81 @@
+scriptencoding utf-8
+
+let s:spc = g:airline_symbols.space
+
+function! s:SetDefault(var, val)
+  if !exists(a:var)
+    execute 'let ' . a:var . '=' . string(a:val)
+  endif
+endfunction
+
+" Left and right delimiters (added only when status string is not empty)
+call s:SetDefault( 'g:airline#extensions#vimtex#left',       "{")
+call s:SetDefault( 'g:airline#extensions#vimtex#right',      "}")
+
+" The current tex file is the main project file
+call s:SetDefault( 'g:airline#extensions#vimtex#main',       "" )
+"
+" The current tex file is a subfile of the project
+" and the compilation is set for the main file
+call s:SetDefault( 'g:airline#extensions#vimtex#sub_main',   "m")
+"
+" The current tex file is a subfile of the project
+" and the compilation is set for this subfile
+call s:SetDefault( 'g:airline#extensions#vimtex#sub_local',  "l")
+"
+" Compilation is running and continuous compilation is off
+call s:SetDefault( 'g:airline#extensions#vimtex#compiled',   "c₁")
+
+" Compilation is running and continuous compilation is on
+call s:SetDefault( 'g:airline#extensions#vimtex#continuous', "c")
+
+" Viewer is opened
+call s:SetDefault( 'g:airline#extensions#vimtex#viewer',     "v")
+
+function! airline#extensions#vimtex#init(ext)
+  call airline#parts#define_raw('vimtex', '%{airline#extensions#vimtex#get_scope()}')
+  call a:ext.add_statusline_func('airline#extensions#vimtex#apply')
+endfunction
+
+function! airline#extensions#vimtex#apply(...)
+  if exists("b:vimtex")
+    let w:airline_section_x = get(w:, 'airline_section_x', g:airline_section_x)
+    let w:airline_section_x.=s:spc.g:airline_left_alt_sep.s:spc.'%{airline#extensions#vimtex#get_scope()}'
+  endif
+endfunction
+
+function! airline#extensions#vimtex#get_scope()
+  let l:status = ''
+
+  let vt_local = get(b:, 'vimtex_local', {})
+  if empty(vt_local)
+    let l:status .= g:airline#extensions#vimtex#main
+  else
+    if get(vt_local, 'active')
+      let l:status .= g:airline#extensions#vimtex#sub_local
+    else
+      let l:status .= g:airline#extensions#vimtex#sub_main
+    endif
+  endif
+
+  if get(get(get(b:, 'vimtex', {}), 'viewer', {}), 'xwin_id')
+    let l:status .= g:airline#extensions#vimtex#viewer
+  endif
+
+  let l:compiler = get(get(b:, 'vimtex', {}), 'compiler', {})
+  if !empty(l:compiler)
+    if has_key(l:compiler, 'is_running') && b:vimtex.compiler.is_running()
+      if get(l:compiler, 'continuous')
+        let l:status .= g:airline#extensions#vimtex#continuous
+      else
+        let l:status .= g:airline#extensions#vimtex#compiled
+      endif
+    endif
+  endif
+
+  if !empty(l:status)
+    let l:status = g:airline#extensions#vimtex#left . l:status . g:airline#extensions#vimtex#right
+  endif
+  return l:status
+endfunction
+