Support for historical usernames with trailing hyphens.

[En-En-Code]

By switching to a negated look-ahead assertion, the new RegExp can easily detect double hyphens while also allowing a single hyphen at the end of a username. We don't lose the following character checked against [a-z\d] from the positive look-ahead assertion as this is covered by the earlier instance of this in the expression.

Added new unit test cases and updated README.md.

Resolves #4.