Allow specifying RSpec command line options

[brynary]

Spin will ignore any arguments passed to spin serve that are not paths to files:

https://github.com/jstorimer/spin/blob/72fbfc6ab6a1de48ffdcd0229488649f7ec4da05/bin/spin#L191

This is unfortunate, as it prevents passing RSpec options to spin serve. I propose spin support a way to send in ARGV options that get set as the ARGV in the forked process.

Maybe we could use this style:

spin push -- --color --profile --tag type:model

Thoughts?

[jstorimer]

Makes sense to me. I like it.

[andhapp]

Is specifying rspec options in .rspec file and letting spin pick it up from there not the best option? or are you suggesting that spin should support both.

[rosic]

I've cleaned up the bits for test unit from PR #45 to support this syntax with Rspec as well.