Allow use of custom parameters on rtm_connect()

[kamushadenes]

Summary

This PR allows the use of additional parameters on rtm_connect().

With this patch, when rtm_connect() is invoked like:

self.slack.rtm_connect(no_latest=True, no_unreads=True, presence_sub=True)

a huge performance improvement was seen in bot general responsiveness.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #210 into master will decrease coverage by 0.06%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
- Coverage   59.67%   59.61%   -0.07%     
==========================================
  Files           9        9              
  Lines         310      312       +2     
  Branches       57       57              
==========================================
+ Hits          185      186       +1     
- Misses        111      112       +1     
  Partials       14       14

Impacted Files	Coverage Δ
slackclient/server.py	59.05% <100%> (ø)	⬆️
slackclient/client.py	37.93% <50%> (+0.43%)	⬆️

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[aoberoi]

thanks for the PR! i think these performance gains (and more) can be realized by the change in #209 by using the rtm.connect Web API method. i still like this idea because it will allow developers to be more specific about which tradeoffs they need. i'd like to see this land, but i also want to hear what @Roach has to say about it.

[Roach]

I think this is definitely a valuable change, and I'll be merging this as soon as I get #209 sorted. Ideally, We'd like to allow the developer to specify rtm.start or rtm.connect and pass these vars into that call to provide additional configurability. 👍

[Roach]

Fixed merge conflicts with updated master

[Roach]

@kamushadenes can you sign the CLA real quick so I can merge this change?
https://cla-assistant.io/slackapi/python-slackclient?pullRequest=210

[kamushadenes]

Guys, REALLY SORRY for the massive delay.

I had a busy month and totally forgot to check GitHub.

I signed the CLA now.