Add basic thread support

[pmacg]

This change adds some very basic thread support to the application:

• If a message in a channel has replies, show the number of messages in the thread
• Pressing 't' on a particular message shows a thread view with just the messages in the thread
• Pressing 't' on a message in a thread returns to the channel
• Posting a message while viewing a thread will post the message in the thread rather than in the channel

Resolves Issue #125 .

[pmacg]

Actually addresses #105

[k4ml]

Seem to work so far. This PR prevent me from going away ;)

[mitas]

@k4ml me too

[icyJoseph]

@pmacg I too vouch for this PR, seems to work swell. @haskellcamargo any comments?

[haskellcamargo]

I'm gonna take a look on this PR tomorrow! I'm excited for thread support as well.

[jtagcat]

It is April, Slack has rolled out it's new slower than ever UI. I can see the last word being typed while typing the next word. Any updates?

[ubuntudroid]

Is this still being worked on?

[pmacg]

Is this still being worked on?

I'm not still working on it and I actually no longer use slack, so I won't know if it breaks!

The PR is just waiting for approval from @haskellcamargo

[james-see]

@haskellcamargo anything? can we merge if it works?

[duynguyenhoang]

Thank you for this effort, I will test and merge it ASAP. Sorry I am quite busy recently and unable to check all PRs

[jotech]

any updates on this? I checked the pull request locally and it seems to work with base branch!

[satelliteoflove]

@duynguyenhoang do you have any updates on this? Many thanks.