PSR-15 support

[ciaranmcnulty]

If someone wants to implement this...

• [ ] Make an App and AppFactory (like https://github.com/ciaranmcnulty/behat-psr7extension/tree/master/src/ZendExpressive)
• [ ] Register the services (in https://github.com/ciaranmcnulty/behat-psr7extension/tree/master/src/ServiceContainer)
• [ ] Add a section to README.md
• [ ] Update the Behat suite to include some new tests

[ciaranmcnulty]

Also this can be higher priority than Zend and Slim in case they end up implementing the PSR soon

[cherifGsoul]

Now, this should be implemented for Mezzio, right?

[ciaranmcnulty]

Go for it :) I’m not using Mezzio lately but PRs are welcome