Make Konacha grep aware

Konacha loads all iframes even when running a single test.

This PR checks If `?grep` parameter is passed, and loads a single Iframe for the spec who's file name is included in the `grep` parameter.

Of course this requires naming convention (naming the file name same as the name of the spec).  That's why it's disabled by default, and if someone wants to use it, they can enable it and make sure all file names follow the convention.

The option is `iframe_economy`.

### What encouraged me to do this


I'm building a large Ember.js app with currently 300 javascript files ( that number will easily double when I complete the app )

Due to the large number of JS files and complex dependencies, `javascript_include_tag` takes around 3 seconds.  Multiply that by the number of specs (iframes) and the current Konacha test suite takes more than a minute to load.

Since Konacha loads all iframes when running a single test, I couldn't speed it up by running one test, which made my test suite basically unusable.

This PR doesn't solve the whole issue (i.e. it doesn't speed up running the entire test suite). So if you have any ideas or suggestions, I would be more than grateful. :)