Exclude pyvows and preggy from tracebacks

[Zearin]

When a vow is broken, it prints a traceback. Overall, this is useful information. But the traceback also includes information about how pyvows (and/or preggy) handled the error.

In most cases, this information isn't useful to the person reading the traceback. It just adds noise for the user to sift through, instead of helping them pinpoint why the vow was broken.

Recommendations

I have two ideas for improving this:

• color-code the pyvows/preggy parts of the traceback differently (perhaps with the ANSI code for “dim” text)
• exclude the pyvows/preggy parts of the traceback, unless the user is using -vvvv; (or, perhaps, offer a separate --debug option to toggle the visibility of these portions of the traceback)