Compile all CSS through a main LESS file

Here it is the other suggestion.

- Removed all styles from `index.tmpl`, now they are in `styles/mdwiki.css`.  
- `styles/main.less` require all LESS and CSS files in the proper order. It's suggested to include future additions in this file.
- An empty `style/custom.css` for non advanced users to place their custom CSS without the need of more knowledge.