Add auto fetching for new css files in themes/

**main.js**
Added functionality to by-pass the "Add it to main.js in the Themes.allThemes array" part in your original instructions. Once you add a css file to the themes/ directory, no other steps would be required. Other than reloading or restarting Brackets. This may not be the best solution but I would like to know what you think. 

**README.md**
Fixed spelling.

I love the fact that your extension is very clean, lightweight and does its task without excess. I tried to keep my additions minimal, following your approach. 

Thanks,
John