Add styling for Linter package

The ubiquitous [Linter] package is indispensable, but its default styling is a harsh contrast to Solarized: 

![default-linter-styling](https://cloud.githubusercontent.com/assets/148868/5218767/4fcd7534-761e-11e4-8716-04923004f694.png)

Styling Linter hints with Solarized colors lets warnings and errors stand out legibly and harmoniously: 

![solarized-linter-styling](https://cloud.githubusercontent.com/assets/148868/5218797/a0e6fcba-761e-11e4-86be-7b9dd385cda0.png)

[Linter]: https://github.com/AtomLinter/Linter