Allow multiple prefix patterns.

Takes advantage of the changes made to the es6_module_transpiler-rails
[here](https://github.com/dockyard/es6_module_transpiler-rails/pull/3).

Maintains backwards compatibility with prior
`config.ember_appkit.prefix_pattern` setting.

Depends on `es6_module_transpiler-rails` > 0.1.0 (needs to be published).

Closes #26.