v0.21

This release allows stripping selected annotations and passing custom headers to HTTP requests send to Alertmanager.

Full changelog:

• #368 feat(backend): add support for custom headers (#368) (@ephur)
• #369 feat(docker): build and deploy docker images from Travis CI (@prymitive)
• #366 feat(build): use Go modules to install all cli tools (@prymitive)
• #365 feat(backend): allow stripping annotations (@prymitive)
• #354 chore(api): drop counters from API response (@prymitive)