Add timed events and callbacks.

[SaschaJust]

Discordify should support timer and corresponding callbacks to report the state of long running tasks. This would be useful in progress monitoring.

There are two things that need to be added for supporting a command that checks the progress.

• Means to spawn a thread that calls this command periodically based on a given timer.
• Processing of the output of the command and firing of the corresponding payload.

Example

# Reports the progress every 5 minutes
discordify --timer 300 --progress 'du -sch results' my_tool

@nicbet thoughts?

[nicbet]

I think it would be helpful for discordify to periodically emit the past n lines of stdout / stderr buffers of long running tasks.

[SaschaJust]

Buffer size currently is 5, but we can make this configurable while implementing this feature. Tracking in #4