Changes in 0.2.0

Added:

Handling for several new event types:
- Pull request reviews (#20)
- Pull request assignments (#23)
- Pull request labels (#23)
- Pull request milestones (#35)
- Issue & pull request title edits (#36)
- Releases (#21)
Handling of draft pull requests (#34)
Handling for draft pull requests marked ready for review (#34)
Converting :emoji: to Unicode emoji in most text output (#17)
- Requires optional dependency: https://pypi.org/project/emoji/

Changed:

secret setting is now called client_secret (#42)
- The plugin will automatically migrate configs to use the new name, but
  manual editing is encouraged in advanced setups (e.g. if the user Sopel
  runs as cannot write to the config file)
Quoted text is ignored when generating comment output (#41)
Unicode ellipsis now used instead of three periods (#31)
Oxford comma now used in generated lists (#40)
Webhook payload processing is handed off to a separate thread (#22)
- Bottle can handle only one request at a time; doing this should minimize
  the potential for failed deliveries, as GitHub's webhook delivery service
  times out very quickly

Fixed:

GitHub URL regexes should match all possible usernames & repo names (#38)
- Of particular note: dots (.) are now recognized in repository links,
  such as Sopel's website repo: https://github.com/sopel-irc/sopel.chat

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