Add :Debug command

The :Debug command quickly starts a new debugging sessions.

* adds the `termdebug` package
* invokes `TermdebugCommand` with the given arguments

autoload/do.vim

@@ -68,3 +68,13 @@ function! do#rename_include_guard(old)
   " Jump back to the set mark
   normal 'r
 endfunction
+
+" Setup and start a debugging command.
+function! do#debug(...)
+  packadd termdebug
+  let l:command = 'TermdebugCommand'
+  for l:arg in a:000
+    let l:command = l:command.' '.l:arg
+  endfor
+  exec l:command
+endfunction

plugin/commands.vim

@@ -14,4 +14,7 @@ command! -nargs=1 TabWidth call do#set_tab_width(<f-args>)
 command! ToggleCheckbox call do#toggle_checkbox()
 
 " Show highlight groups under the cursor
-command! CursorHighlightGroups :call do#cursor_highlight_groups()
+command! CursorHighlightGroups call do#cursor_highlight_groups()
+
+" Setup and invoke a :TermdebugCommand
+command! -nargs=+ -complete=file Debug call do#debug(<f-args>)