Added OptionsBuilder and fixed anchorMenu with small screens

This pull request adds:
- New OptionsBuilder class used to configure the Builder. Contains new ```apiName``` and ```apiDescription``` parameters, displayed at the top of the generated HTML.
- The ```anchorMenu``` on the left is now fixed. The affix behavior only happens when the screen width is equal or greater than 991px (```col-md``` media queries in Bootstrap).

The README has been updated with this new behavior.