Add option to process files with a custom function

The `process` option can now take a function which will be called once for each file. The returned value will be used as the source code.

Here's my use case:

```js
// Replace all 'use strict' statements in the code with a single one at the top:
runt.initConfig({
  concat: {
    dist: {
      options: {
        banner: "'use strict';\n",
        process: function(src, filepath) {
          return '// Source: ' + filepath + '\n' +
            src.replace(/(^|\n)[ \t]*('use strict'|"use strict");?\s*/g, '$1');
        },
      },
      files: { ... },
    },
  },
});
```