Lexer for Agda

[ionathanch]

[Issue #709] Based on Agda's lexer, I've filled out the remaining bits of the Agda lexer. It doesn't quite match the behaviour, especially in places where the semantics of the keywords matter (e.g. imports, pragmas), but this should be sufficient to cover most use cases and look nice, and I think Rouge should have at least some Agda support with improvements as needed later on. (And thanks @ayberkt for the initial work!)

[awsloth]

Not sure the etiquette on this, but I would like to boost this. The support for agda here would be great!