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