Use JSON instead of YAML for configuration file

[lencioni]

This projected is targeted at folks working near the front end. Besides being a better format IMHO, I think that JSON will be more familiar to these people. I think we should switch to JSON to lower the barrier to entry, just a little bit.

[trotzig]

Sounds like a good idea, though we have to figure out how to do work around doing dynamic configuration like this:

port: <%= 7654 + ENV['EXECUTOR_NUMBER'].to_i %>
source_files:
  - http://ajax.googleapis.com/ajax/libs/jquery/2.1.3/jquery.min.js
  - public/assets/diffux_ci.js
s3_access_key_id: <%= ENV['S3_ACCESS_KEY_ID'] %>
s3_secret_access_key: <%= ENV['S3_SECRET_ACCESS_KEY'] %>
s3_bucket_name: diffux_ci-diffs
snapshots_folder: ./tmp/diffux-snapshots-ci

This is what .diffux.yaml looks like for us at the moment.

[lencioni]

It might be worth waiting for #81 and then using JavaScript for the configuration. This is the approach we are landing on in import-js.

[lencioni]

It's worth noting that in #92 we took some steps toward making it so we don't need the interpolation in the YAML. We could do similar things for remaining options.

[trotzig]

This is handled as part of the rewrite to javascript. Closing since we're closing in on a release of 3.0.