ntfy-alertmanager v0.3.0

This release contains a lot of internal refactoring/improvements and
adds support for newly released ntfy features.

Features:

- Add support for ntfy access-tokens and message icons, e-mails,
  phone calls
- A persistent cache with redis

Changes:
- By default grouped alerts will no longer be split apart (alert-mode
  multi) and the cache is deactivated (type disabled). With these
  changes the behaviour of this bridge is closer to what is configured
  in Alertmanager/Prometheus.

Thorben Günther (33):
      version: Use linker flag
      Move http.Client to receiver struct
      Add icon support
      Set default header for User-Agent
      tests: Remove comments from test config
      ntfy: Support authentication via access-tokens
      config: Forbid to set a user/password without the other
      Rename receiver to bridge
      Slightly improve logging
      multi mode: Simplify code
      Simplify outgoing Basic Authentication
      Sort labels alphabetically
      cache: Move to own interface package
      config: Add cache type
      cache: Support redis
      config: Add redis-url
      docker: Add redis section to docker-compose file
      readme: Add more contribution links
      Add ntfy email support
      Upgrade dependencies
      Add phone call support
      publish: Try to decode ntfy error string
      cache/redis: Try to ping redis on cache creation
      config: Move to own package
      cache: Move creation logic into cache package
      cache: Add a "disabled" cache
      config: Switch default values of alert mode and cache type
      http: Use own multiplexer
      http: Use own server
      cache/redis: Rig up timeout with context
      Implement graceful shutdown
      readme: Move example config out
      Simplify HTTP Basic Authentication middleware