Add support for triple ///, //! and /*! style comments + basic Rust support

This adds support for:

- rust-style `///` triple slash docblocks (also fixes #214).
- rust `//!` and `/*!` comments for documenting modules.
- adds a very dumb JsdocsRust class that just suppresses all docblock generation because we don't have any of that in rust.

I hope it's all reasonable changes :)