Allow underscores to appear in Harbormaster variable names

Summary:
See PHI859. Ref T13195. The regexp for replacing variables currently does not include underscores.

Include underscores.

I also made a note in T13088: we should (almost certainly?) throw immediately if you try to pass a bogus variable name as a custom parameter, but this is a slightly larger change.

Test Plan:

• Made an "http request" build plan with ?x=${initiator.phid}&y={$some_variable}.
• Added some_variable as a parameter to the parameter collection.
• Before patch: initiator.phid was replaced, but some_variable was not.
• After patch: both variables are replaced.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13195

Differential Revision: https://secure.phabricator.com/D19645