add bridgy publish support. https://www.brid.gy/about#webmentions

cc @dshanske, @pfefferle
fix mf2 class: u-syndication, not usyndication. http://indiewebcamp.com/rel-syndication
bridgy publish hook: check that post_ID is non-null
bridgy publish: don't show empty list when there are no syndication links
Merge pull request #3 from snarfed/master

add bridgy publish support, fix u-syndication class