Improve overall functionality

* For non `.css` files, add the ability to run the selected text through autoprefixer.
* Handle errors, and instead of showing the developer tools, make Atom beep when something went wrong.

  ![](https://f.cloud.github.com/assets/1223565/2282084/38f3732e-9fa8-11e3-9511-fdcfbe8610a7.gif)

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Thank @mathiasbynens for the "invite" :)