Add ability to eval Lua code

[dcampos]

This pull request adds the ability to write Lua code directly in eval blocks, without the need to call luaeval() from Vimscript:

snippet today
    Today is `!lua os.date('%Y-%m-%d')`

The !lua part can be abbreviated to !l.

It will also make it possible to write !vim or !v to mean Vimscript, which is the default when no language is indicated.