Add basic support for units

Test Plan:
test.html
update .arcconfig to https
Basic unit support.

Test Plan:
test.html

> Tests completed in 2316 milliseconds.
> 966 tests of 966 passed, 0 failed.

Reviewers: aria

Subscribers: alex

Differential Revision: https://phabricator.khanacademy.org/D14862
Add / fix more tests.

Two big changes here:
* in `compare` we make sure the things have the same units (still some
  more work to do this properly)
* we no longer allow prefixes on every unit

Test plan:
KAS Browser Tests! (one still fails)

Auditors: alex
Add unit parser.

It makes things simpler all around for the unit parser to live in KAS
rather than perseus.

Test plan:
KAS Browser Tests: 969 test of 969 passed, 0 failed.

Auditors: alex