Add two options, g:bufferline_sort_function and g:bufferline_hide_active_buffer

I'm loving bufferline, but wanted to tweak a couple things.  

I use the [bufstop](https://github.com/mihaifm/bufstop) plugin which I love, and I wanted bufferline to show the buffers in the same order that bufstop uses for its speed toggle.  So, being able to specify a custom sort function made that possible.

Then I added an option to not show the active buffer. My terminal windows aren't usually very wide, and since I already have the name of the file in my status bar (I put bufferline in the command bar), I didn't need it doubled up.  So, this allows you to hide the active buffer from the bufferline list.