add -retainids to `gt merge'
accept and ignore empty input GFF3s in tidy modes
gtruby: fix FeatureNode.set_source
properly initialize string buffers

Closes #282.
warn when weird characters appear in argv

Closes #241.
Update README.md

add link to gitter chatroom
closes #230
Merge pull request #283 from satta/sanger

Some bugfixes and minor feature additions
Merge pull request #284 from Garonenur/gitter

Update README.md