Implement block-alternate origin path

This is fixing #63 by adding a way to alternate, at block-level, the origin path, when preparing concat and uglify config.

Usually, when something like that is specified in, let's say `app.index.html`:

```
        <!-- build:js scripts/main.js -->
        <script src="scripts/foo.js"></script>
        <script src="scripts/bar.js"></script>
        <!-- endbuild -->
```
`foo.js` and `bar.js` are searched relatively to the file (i.e. uglify and concant config are trying to get file from `app/scripts`).

By specifying an alternate directory (for example `.tmp`), we can change that:

```
        <!-- build:js(.tmp) scripts/main.js -->
        <script src="scripts/foo.js"></script>
        <script src="scripts/bar.js"></script>
        <!-- endbuild -->
```
foo.js` and `bar.js` are searched relatively to the `.tmp` directory (i.e. `.tmp/scripts/foo.js`).