Icon

Provide a simple function for developer to render icon.

- `at_icon($name, $source)`
- Support fontawesome
- Support fontello
- Provide `drush fontello` to build fontello
- Two unit-test cases, one for fontawesome, one for fontello

### Ref

- https://github.com/v3knet/at_base/blob/route-editable/at_base.module#L227
- https://github.com/v3knet/at_base/blob/route-editable/config/require.yml#L30