Add completions and documents

Use `vimdoc .` to generate documentation.
User can use plugin manager to do it automatically when update plugin.

autoload/requirements/utils.vim

@@ -0,0 +1,11 @@
+""
+" @section Configuration, config
+
+function! s:Flag(name, default) abort
+    let l:scope = get(split(a:name, ':'), 0, 'g:')
+    let l:name = get(split(a:name, ':'), -1)
+    let {l:scope}:{l:name} = get({l:scope}:, l:name, a:default)
+endfunction
+
+let g:requirements#utils#plugin = {'Flag': funcref('s:Flag')}
+" vim: et sw=4 ts=4 sts=4: