Use templates for javascript, css, etc.

[saimn]

Sometimes it would be useful to be able to use Jinja templates for javascript or css files.
An example use case is #245.

[anarcat]

this looks similar to #74 ...

[saimn]

It is close indeed. I was not sure at that time (and I'm still not) if it is a good idea to pass all the files from a theme through Jinja. Need more thoughts.