Add thinca/vim-fontzoom

Added [`thinca/vim-fontzoom`](https://github.com/thinca/vim-fontzoom) as currently there seems to be no super easy way to control the font size. Typical use cases include presentations.

vim-fontzoom includes a few default keymappings (`+`, `-`, `ctrl-scroll`), I added equals (`=`) mapped to reset. Vim 7.4 cannot map `ctrl-plus/minus/0` so the obvious mappings are ruled out. Let's hope Vim 8 changes this.

I also evaluated [`drmikehenry/vim-fontsize`](https://github.com/drmikehenry/vim-fontsize), but didn't like the separate font sizing mode. Others were more general font managers and those are overkills in my opinion. [`thinca/vim-fontzoom`](https://github.com/thinca/vim-fontzoom) seemed to be the best out there.

In an ideal situation, the window size of gVim wouldn't change, but I guess we have to live with this limitation.

Tested on Windows 8 x64 with vim 7.4. I guess someone should test this on some Linux distro before merging.