v1.3.0: Support custom forbid reason messages (#11)

In order to allow users to communicate intent to collaborators,
optionally embed custom messages into each forbidden pattern.

The syntax is as follows: `identifier(# message goes here)?`

Example: `^fmt\.Errorf(# Please don't use this!)?$`

Regular expressions containing custom messages are effectively identical
to ones that don't, because the sub-expression containing it is marked
optional with a `?`. All this commit does is parse out any recognized
custom message, and place it prominently in the tool's output. The
regular expression itself is omitted from the tool's output when a
custom message is specified.