Add option to use a custom sorting function

This allows you to change the order the buffers are listed in. For
example, if you wanted to sort them by most recently used.
Add option to hide the active buffer from the list

This allows you to save space when space is at a premium
Fix typo
Merge pull request #17 from bentomas/master

Add two options, g:bufferline_sort_function and g:bufferline_hide_active_buffer