Show excerpts instead of long posts for all sources.

Instagram and Flickr posts occasionally have long descriptions that we also want to truncate. Tweets shouldn't ever be longer than 140 characters, but Twitter could change the length at some point in the future, and it's also good to have consistency across all the sources.


git-svn-id: https://plugins.svn.wordpress.org/tagregator/trunk@1008877 b8457f37-d9ea-0310-8a92-e5e31aec5664