feat: add support for more expressive handlers.

[samcamwilliams]

This commit adds support for:

• Handlers.once: A method of adding a single-run handler that is removed after exection.
• Optional MaxRuns argument for each handler, letting you choose the number of runs before it removes itself. Default is infinite.
• More expressive and easy to use matches. As well as functions, handlers now supports: -> Strings as match, checking against the Action tag. -> 'MessageMatchSpecs': Partially described messages. In this mode, each tag given in the 'spec' is checked against the value in the message. It allows you to:
  1. Wildcard: Is the tag given at all?
  2. Function: Run a function against the tag value (optional second argument is the full message)
  3. String: Exact match
  4. String: Lua matching syntax