Add 'process' option for preprocessing source files

This adds support for preprocessing templates before neuter get's a chance to do its thing. It's modeled after the option of the same name in [grunt-contrib-concat](https://github.com/gruntjs/grunt-contrib-concat#process).

Preprocessing templates enables conditional requires based on the environment, e.g.

```javascript
// {% if (env.DEBUG) { %}
require('debug-version');
// {% } else { %}
require('release-version');
// {% } %}
```