add Go support to prism

The author of the other [Go-inclusion-PR](https://github.com/LeaVerou/prism/pull/125) doesn't seem to respond.
This is my attempt.

I wasn't sure with the matching categories.
Go has keywords and [predeclared identifiers](http://golang.org/ref/spec#Predeclared_identifiers) for types, functions, constants and nil.
For now, I added the predeclared stuff minus `true` and `false` into builtin.