Add ability to manually trigger completion menu (#2177)

* Add ability to manually trigger completion menu
* Mention :ALEComplete in completion docs
* Add test for ALEComplete

plugin/ale.vim

@@ -204,6 +204,8 @@ command! -bar ALEDocumentation :call ale#hover#ShowDocumentationAtCursor()
 " Search for appearances of a symbol, such as a type name or function name.
 command! -nargs=1 ALESymbolSearch :call ale#symbol#Search(<q-args>)
 
+command! -bar ALEComplete :call ale#completion#AlwaysGetCompletions()
+
 " <Plug> mappings for commands
 nnoremap <silent> <Plug>(ale_previous) :ALEPrevious<Return>
 nnoremap <silent> <Plug>(ale_previous_wrap) :ALEPreviousWrap<Return>
@@ -229,6 +231,7 @@ nnoremap <silent> <Plug>(ale_go_to_definition_in_vsplit) :ALEGoToDefinitionInVSp
 nnoremap <silent> <Plug>(ale_find_references) :ALEFindReferences<Return>
 nnoremap <silent> <Plug>(ale_hover) :ALEHover<Return>
 nnoremap <silent> <Plug>(ale_documentation) :ALEDocumentation<Return>
+inoremap <silent> <Plug>(ale_complete) <C-\><C-O>:ALEComplete<Return>
 
 " Set up autocmd groups now.
 call ale#events#Init()