Unicode mathematical notation using `conceal`

Inspired by (and based on) [vim-cute-python](https://github.com/ehamberg/vim-cute-python).

After enabling `g:coffee_cute`, certain keywords are replaced with their equivalent mathematical notation (such as ∈, ∧ and ∨) and function arrows are converted to unicode as well.

CoffeeScript is pretty terse already, so I doubt anyone will use this, but it's a nice showcase of `conceal`, don't you think?