[POC] Draft of a digest formatter

After reading https://github.com/Behat/Behat/issues/337,
I came up with the idea of implementing that formatter to get a list of scenarios.

My goal is to make it available through `behat --digest` (like `behat --config-reference`) and to make it usefull to quickly find a scenario file/line to execute this particular one.

For the moment, one must use `behat --dry-run -f digest`
Here's the result:
![screenshot from 2014-09-15 00 14 38](https://cloud.githubusercontent.com/assets/668604/4265727/ca40761e-3c5c-11e4-98a6-2d13357c566a.png)

In my opinion, if you think it's better to keep it as a formatter (than a --digest option), then test result should be displayed next to the scenario title with a tick, an "X", or anything that show the actual scenario execution result (one might still run with --dry-run to quicky view the list).

This is open to discussion, I used "digest" instead of "terse" cause it simply makes more sense to me.