Support private channels

[insomniacslk]

This patch adds support to private channels. Previously, private
channels were read-only, while now it's bidirectional like public
channels and IMs.

This required a refactoring of the channels API, and a few bug fixes.
Now anything channel-related goes through the Channel and Channels
structures, and should not use the Slack conversations API directly
anymore.

Signed-off-by: Andrea Barberio insomniac@slackware.it

[insomniacslk]

CC @gjabell

[codecov]

Codecov Report

Merging #165 (71a4684) into master (3cfeb31) will decrease coverage by 0.85%.
The diff coverage is 5.26%.

@@            Coverage Diff            @@
##           master    #165      +/-   ##
=========================================
- Coverage    9.84%   8.99%   -0.86%     
=========================================
  Files           9       9              
  Lines        1026    1090      +64     
=========================================
- Hits          101      98       -3     
- Misses        921     987      +66     
- Partials        4       5       +1     

Flag	Coverage Δ
unittest	8.99% <5.26%> (-0.86%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/ircslack/event_handler.go	0.00% <0.00%> (ø)
pkg/ircslack/irc_context.go	0.00% <0.00%> (ø)
pkg/ircslack/irc_server.go	0.00% <0.00%> (ø)
pkg/ircslack/channel.go	6.77% <6.77%> (ø)
pkg/ircslack/users.go	43.75% <8.82%> (-11.81%)	⬇️
pkg/ircslack/channels.go	37.63% <13.20%> (-32.58%)	⬇️

---

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[insomniacslk]

TODO before merging this PR: now the channel names in the chanlist don't show have the prefix anymore (e.g. general instead of #general)

[insomniacslk]

Tested on multiple teams on public and private channels, and on IM. Still not working in threads

[insomniacslk]

Now also tested on threads in public and private channels

[insomniacslk]

Messages and threads to self are not yet supported. See #166

[insomniacslk]

Will merge this PR tomorrow if no other issue appears

[insomniacslk]

the last update fixes IMs that I broke accidentally in this PR. So now the test matrix is:

	public channel	private channel	multiparty IM	IM
from me	works	works	doesn't work (roadmap)	works
to me	works	works	works	works
thread from me	works	works	untested	doesn't work (roadmap)
thread to me	works	works	untested	works (sends in the IM chat)

[insomniacslk]

Updated readme with feature matrix

[insomniacslk]

Merged! This one took quite a lot of work, but I'm happy with the result (hopefully I won't take those words back :D ). Next is to add some serious testing, since any change to this project is hard to validate.