Import USA.gov domain list

Fixes #23 via [this import script](https://github.com/benbalter/gman/blob/usagov-domain-list/script/vendor-us).

* Vendors the USA.gov-maintained list of US domains into domains.txt
* Normalizes and cleans inputs
* Validates domains
* Rejects academic domains
* Sorts
* Ensures uniqueness, and
* Merges into the existing `lib/domains.txt` list

**Updating:** `script/vendor-us` (that's it... will automatically fetch latest version of the list and merge. You can check for changes and commit via `git status`. It's also probably a good idea to run `script/ci-build` for good measure.)

Thanks to @afeijoo and @ErikSArnold for reaching out and publishing the list over at https://github.com/GSA-OCSIT/govt-urls.

/cc @gbinal

:us: :us: :us: