Added feed summary plugin.
This plugin allows to use the `Summary:` metadata as feed contents.
Merge pull request #113 from florianjacob/feed_summary

Add feed summary plugin
Remove gzip references in Assets plugin README

gzip filter removed in:
https://github.com/miracle2k/webassets/commit/d0fb2414208353625ee04cf236e6961efdff8f59

Fixes #115