Add name to filetype so it can be ignored by other plugins more easily

[axsaucedo]

Currently the buftype is nofile which makes sense, it would also help if the filetype was set to contextvim or similar so it's possible to explicitly ignore by other plugins. This would also allow simple commands that allow for cursor to be moved away from buffer.

This will help as well as I've noticed that if you open and close nerdtree/nvimtree too fast the cursor gets placed inside the context.vim window which is quite annoying as re-opening the tree again causes weird issues.