Add global BufstopGetBufferInfo function

I'm writing a plugin (https://github.com/bentomas/bufstop_tabline) that needs access to the bufstop buffer info, so this commit adds a global function to get it.
Merge pull request #11 from bentomas/master

Add global BufstopGetBufferInfo function
Consistent color settings fixes #12
Added g:BufstopSorting option and various fixes