Create a separate repo to host Vim grammar to enable pulling it from Vundle

[Vundle](http://github.com/VundleVim/Vundle.Vim), the plug-in manager for Vim, needs a repository with a grammar on Github to be able to use it as a plug-in. There are several repos on Guthub with ISPC grammar (for example, this one [https://github.com/Twinklebear/ispc.vim](https://github.com/Twinklebear/ispc.vim)). We need an official one.