added --api option to exercism configure; updated tests

[morphatic]

In response to #145

Summary of changes:

• add --api and -a parameters to exercism configure
• updated config_test.go to test success of changes

[morphatic]

Per conversation in #146 I also added a couple more steps in the Development section of the README file to clarify the process for contributing developers.

[kytrinyx]

This is great! Thanks so much for jumping in and getting this done so quickly. I'll get cut a new release tonight or tomorrow morning.

[kytrinyx]

I've published a new release, but only here on github (I've not updated the homebrew repository, I sometimes skip patch releases there). Let me know if it would be easier on your students if there's a homebrew installation option.

[morphatic]

Thank you! My class starts on Monday and I'll have 52 brand new recruits, the majority of whom have never written any code at all. I don't think many of them will be savvy enough to realize how cool the homebrew option is. It will be good for them to follow the instructions for manual installation. :)

[kytrinyx]

It will be good for them to follow the instructions for manual installation. :)

No doubt :) There's so much implicit knowledge in getting something like that working. I really had no idea until I tried to explain it to beginners.