Add Rust-specific support.

- Add support for coloring of Rust-specific items such as attributes,
  macros, module paths. The scopes are based on Rust Sublime package:
  https://github.com/jhasse/sublime-rust
Add screenshot for Rust.
Change html highlighting so it is less like monokai
Change image for html
Prepare 0.6.0 release
Change scss image
Rebuild atom theme
Prepare 0.7.0 release