allow hyphens in usernames

[nathantypanski]

Previously, GitHub usernames containing hyphens were broken: if a
hyphenated username were provided in a r=some-body request to bors,
bors would interpret that request as r=some.

The reason for this is the regex parsing the username in those comments
was only looking for a word (\w+, or equivalently [a-zA-Z0-9_]+),
and words end at hyphens.

This changes those regular expressions to look for any of the word
characters and additionally a hyphen character ([a-zA-Z0-9_-]+).

I am not able to test this in production (I don't have the configuration)
but my experiments at the interpreter suggest this regex is correct for
parsing usernames with hyphens.

>>> import re
>>> c = 'r=chris-morgan'
>>> [m.group(1)
     for m in [re.match(r"r=([a-zA-Z0-9_-]+)", c)]
     if m]
['chris-morgan']
>>> s = 'r=chris-morgan b2f6b40'
>>> [m.group(1)
     for m in [re.match(r"r=([a-zA-Z0-9_-]+) ([a-z0-9]+)", s)]
     if m]
['chris-morgan']

But if anyone has a good clue how to check if this works, let me know.

Closes #38.

[brson]

Thanks! Note that Rust is using an out of date bors and will not get this change until some maintenance is done to bring our infrastructure up to date.