Directionality check for Username profiles

[legastero]

The Username profiles are currently always applying the Bidi rule, but the RFC says the Bidi rule only needs to be satisfied if there are any RTL characters present.

Right now, a username that looks like "5example" will fail because the leading 5 violates the first condition of the Bidi rule (it has a Bidi class of EN, not L, R, or AL).

[ezzatron]

Thanks for the heads up! Should be fairly simple to fix, but I don't have a lot of spare time for this project at the moment. If you need it anytime soon, a PR is your best bet.

Side note, there are still some serious issues with precis-js, brought up in #1, and further documented in #6 and #7. Not related to this issue, but just making sure you're aware if you were looking at using this library in production code. Cheers.