Add build.yml option to disable xcpretty

[jtara]

Rhodes now supports use of xcpretty Gem to clean-up build output for iOS command-line build. If xcpretty Gem is installed, Rhodes build will use it.

However, sometimes you might like to turn it off, to see all of the command-line parameters original error text, etc. It is inconvenient to have to uninstall the xcpretty Gem and the re-install when you want it again.

It would be good to provide some way to disable it, probably a build.yml setting.

Alternately, document any way to disable it otherwise. (maybe xcpretty has some environment variable or dot-file it will look at?)

I've put in a complementary Issue with the xcpretty project. IMO there should be some way to disable globally.

xcpretty/xcpretty#126

My current workaround is to uninstall xcpretty when I want to see everything, then reinstall when I do. Alternately, one could create separate rvm gemsets.

[supermarin]

Addressed your concern in xcpretty/xcpretty#126