Script to list 18F repos with an 18-pages branch

[mbland]

First step towards implementing #51. Prints out all 18F repositories that contain an 18f-pages branch. Right now, there's 61 of them.

Next step would be to check if there's a _config_18f_pages.yml file in the branch that overrides baseurl:. This should be very rare, but nonetheless is necessary for 100% correctness.

After that, we can generate a page in this repo that lists all of the 18F Pages sites.

(This could also become the basis for pruning old directories, via a cron job or something.)

cc: @arowla @ccostino @gbinal @melodykramer

[gbinal]

sounds great to me.

[arowla]

:+1: