Add GetScriptNumber(), upgrade UltiSnipExpand().

Add:
	vimrc
		-Add `GetScriptNumber()`, to retrieve the `<SNR>` of a vim script.

	vimrc_after
		-Reimplement `UltiSnipExpand()` to call `NERD_commenter`'s
		comment-detection local-scope function without a hard-coded `<SNR>`,
		by making use of `GetScriptNumber()`.