Add integration test

[vortizhe]

We must add integration test before publishing the first version.

• Create a phoenix app only in dev and test
• Add hound for integration text with chrome driver
• Test connection and basic actions
  • redirect
  • replace_with
  • append
  • prepend
  • mod_class

[crbelaus]

Since we have to build a Phoenix application to run the integration tests, it may be a good idea to reuse that application as an easy platform where we can implement and try new features.

It could be also very useful to put that application running somewhere public-accessible, so potential users could see Frankt working before trying it in their projects.

As explained in the Phoenix docs, Heroku has some limitations regarding long-lived connections and processes. The most easy alternative seems to be Gigalixir.