Switch line number based on whether a buffer is active

Great plugin! It solves a common vim pain point for me - I want to use relative line numbers while I'm editing a buffer, but often need absolute line numbers for use with external tools like gdb.

This pull request makes it fit my workflow a little better, since I often use gdb in a ConqueTerm inside vim. With this change, switching away from a buffer to a different split (say, from source code to gdb) activates absolute line numbers, making it easy to set breakpoints and such, and switching back reactivates relative line numbers. I think this will be very helpful for others like me and will hurt no one, since relative numbers don't matter much when a buffer isn't active.