ERB support for config file

#51 based off 0.7.0-wip

* [Merge branch 'master' into erb-support](https://github.com/danielbayerlein/tmuxinator/commit/62827f05c3c8a981c4f4ebcc2199fd4058aa1046)
* [Move config_path into block](https://github.com/danielbayerlein/tmuxinator/commit/d3711a17f8ee26cc2da49085f49b0c9b27b0f780)
* [Added ERB support for config file. See #47 and #51.](https://github.com/danielbayerlein/tmuxinator/commit/64afeb64e86055598d9d0fd9d9f36520aa893b34)

@der-flo, @Soliah, @amiel