Add agnoster_beta to support new glyphs

I've fixed the agnoster theme as it currently works with Lokaltog/vim-powerline patched fonts, which has been deprecated, but did not work with the latest version of Lokaltog/powerline patched fonts.

As you can only use one font at a time in your terminal, the current agnoster theme and Powerline for Vim became incompatible without this patch. However, as many people still use the old version of vim-powerline, I created agnoster_beta so people can use whichever they need.

Cheers,

Andrew Schwartzmeyer