Added hashtag-based tiers for payouts, solves #11

Tiers will be decided based on the first hashtag encountered in a commit. If there is no hashtag or the hashtag is not in the list of tiers, the multiplier will be 1 (so fully standard). If there is a hashtag in the commit message that matches the tiers in `config.yml`, that multiplier will be applied to the tip amount. I included in `config.yml.sample` the tiers that were discussed in issue #11, but tiers are fully user-configurable.