Add configurable blacklist phrases and stop words to LinkWithUnclearPurpose

Fixes #94.
I'm not clear on the difference between blacklist phrases and stopwords. Does a blacklist phrase fail links that contain that phrase at all, even if there is other clarifying text? If so, a test that makes that explicit would be helpful:

Here's an example of how I am envisioning it:

link text: "Contains some blacklisted phrase"
blacklisted phrase: "Contains some"
status: "FAIL"