Add "Hobby Plus" pricing tier

From @viktorbenei on Tue Nov 01 2016 02:08:25 GMT-0400 (EDT)

Thanks for your feedback @derrickshowers!

We’re definitely thinking about a pricing plan between free and the PRO ($50) plan, and right now you can get the 1xPRO plan for $25 if you contribute (and maintain) a step to the Bitrise StepLib.

That said, feel free to vote and comment on this feature request, anyone who would like to see a mid tier plan - don’t forget to describe what the limits would be (the way @derrickshowers did)!