Add sort by gist description or file extension alphabetically option.

Added two methods into helpers.py: set_sort_data and sort_gists_data. The first helps prepare the code for proper sorting. It is called inside gists_filter() method for loop. Second method performs the actual sorting logic. It is called at the end of gists_filter() method.

Also have added documentation to README.md and Settings Usage in Gist.sublime-settings.

I am new to python programming so optimizations and/or fixes are greatly appreciated.