Grammars scopes to config + added support to styled-components

[mgtitimoli]

Hi there!

I've been using Styled Components for a while, and recently we decided to add stylelint to our validation pipeline.

In addition to lint from the console, I wanted to get some feedback directly in Atom, and was then when I found this package.

I installed the package, but to my surprise I was not getting any feedback, so after digging a while I noticed that this was caused because the grammar for Styled Components was not included in the currently supported ones.

In order to add a new grammar there were 2 paths to take:

1. Add the new grammar directly to the baseScopes array
2. Promote supported grammars to be a config value, so this way users would be able to specify the grammars they want to lint

After analyzing the pros and cons I ended up picking the nice and marvelous path 2 :sparkles:.

Hope you like this :slightly_smiling_face: