Allow to use a custom function to compile the escape pattern (<%- %>)

[byroot]

EJS.escape_function is now a formatting string.
Example:

EJS.escape_function = '_.escape(%s)'

[byroot]

And since _.escape and friend already take care of the string coercion https://github.com/documentcloud/underscore/blob/master/underscore.js#L1040
maybe we can let this responsibility to escape_function.

[percyhanna]

Wow, I'd love to have this in the gem. Looks great.

[wireframe]

+1

using a custom escape_function really cuts down on our compiled template size. since Underscore is already available in our app just using _.escape vs 6 inline replace statements is a pretty quick win!