fix bugs in PullRequest#score

Score should be zero when at least one [blocker] comment exists.
Ignore comment text inside ~~strike-thru markdown~~.
[blocker] pattern should be case-insensitive.
update README to describe comment parsing rules
make sure threshold configurable

also add settings for future options
implement status check
small refactor
fix handling of multiple strikesthrus

regex was greedy, so everything between first and last strikethru was
being removed

add a spec describing correct behavior
[SB][00000] process strikethrus only once per comment

* The private method comment_score doesn't need to care about blockers.
* Changed has_blocker? and comment_score to take string arguments
  instead of comment objects.
[SB][00000] only count most recent +/-1 per user

Addresses https://github.com/primedia/parliament/issues/6
[SB][00000] refactor #score
[SB][00000] #score should return 0 instead of nil

Was incorrectly returning nil when no +1 or -1 comments present.
[SB][00000] implement required voters
Revert "[SB][00000] implement required voters"

Oops - meant to do this in a branch :(

This reverts commit 83c3ce1607f3b1204b4c65761adab2b379bf61ce.
[SB][00000] implement required voters