add optional alignment arg to draw_text

[ben-wes]

closes #64

only handles the additional argument for vanilla.
opening this PR for further discussion since i'm not sure if that's "the way".

[ben-wes]

added a minor change to achieve the handling for the optional argument with luaL_optinteger.

... while looking at this, i thought that we could similarly use this or luaL_optnumber in other cases for strokewidth and fontsize to make them optional?

EDIT: those additional optional arguments would then possibly look like this: c429057