Remove vim-plug and use minpac

* Add `Pack`, `PackUpdate`, `PackStatus` and `PackClean` commands to
  manage plugins.
* Don't load `minpac` by default.
* Remove `vim-plug` completely.

after/ftplugin/c.vim

@@ -1,18 +1,19 @@
-" Surround visual block in #if 0 block
+" Add Doxygen documentation generation plugin.
+packadd DoxygenToolkit.vim
+nnoremap <leader>d :Dox<CR>
+" Surround visual block in #if 0 block.
 vmap 0 V'<[ ki#if 0<Esc>'>o#endif<Esc>
 " Set 'comments' to format dashed lists in comments.
 setlocal comments=sO:*\ -,mO:*\ \ ,exO:*/,s1:/*,mb:*,ex:*/,:///,://
 " Set 'commentstring' to single line comment style.
 setlocal commentstring=//%s
-" Stop automatic new lines which typing long single liners
+" Stop automatic new lines which typing long single liners.
 setlocal textwidth=0
-" Map K to get YouCompleteMe documentation
+" Map K to get YouCompleteMe documentation.
 nnoremap K :YcmCompleter GetDoc<CR>
 " "ys{motion}t" turns "word" -> "TODO(word):"
 let b:surround_{char2nr("t")} = "TODO(\r):"
 " "ys{motion}/" turns "text" into "/*text*/"
 let b:surround_{char2nr("/")} = "/*\r*/"
-" Map K to get YouCompleteMe documentation
+" Map K to get YouCompleteMe documentation.
 nnoremap K :YcmCompleter GetDoc<CR>
-" DoxygenToolkit
-nnoremap <leader>d :Dox<CR>

after/ftplugin/cmake.vim

@@ -1,3 +1,6 @@
+" Add omnifunc completion plugin.
+packadd vim-cmake-completion
+
 " Enable syntax folding, see .vim/after/syntax/cmake.vim
 setlocal foldmethod=syntax
 " Set comment string

after/ftplugin/cpp.vim

@@ -1,4 +1,5 @@
+" Add Doxygen documentation generation plugin.
+packadd DoxygenToolkit.vim
+nnoremap <leader>d :Dox<CR>
 " Map K to get YouCompleteMe documentation
 nnoremap K :YcmCompleter GetDoc<CR>
-" DoxygenToolkit
-nnoremap <leader>d :Dox<CR>

after/ftplugin/vim.vim

@@ -1,3 +1,6 @@
+" Add omnifunc completion package.
+packadd vimomni
+
 " Mapping for Vim help of the word under cursor.
 nnoremap K :help <C-r><C-w><CR>