Allow to use lulpeg instead of lpeg

[Totktonada]

Don't sure whether it should be runtime autoguessing (pcall(require, 'lpeg'), then pcall(require, 'lulpeg')) or a user provided option.

lpeg did not work (crashes) on my system. This, however, was a long time ago.

[RunsFor]

We switched to lulpeg in #4 which is pure lua, so it should be more stable. One might say she want to use lpeg instead of lulpeg. I see only one major argument for this. It's performance. While it is an absolutely correct wish, it would be tedious to eliminate those "crashes" to make it work stable.

So lets proceed with lulpeg now, and come back to the question "lpeg or lulpeg" later.

BTW, there is also interesting project worth to look at called lpeglabel.