Add functionality to handle variadics/slices (#7)

[Houndie]

Note: Right now the slice will grab every argument from where it is til
the end. This is fine for variadics which must be the last parameter,
but slices don't have to be. Need to implement error handling for
functions which contain a slice argument which is not last.

Alternatively, define alternate syntax for slices such as
"gorram pkg func {some, slice, parameter} other parameters"

[Houndie]

As noted before, I'm not particularly happy with this changeset, it feels very tacked-on to the current code. Doubling the number of converters is probably much cleaner but might be difficult to maintain depending on projected life of the project (if we need to start adding other variations on types, the combinatorics might get high).