include email as user property

[acaire]

• 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.
• I've been mindful about doing atomic commits, adding documentation to my changes, not refactoring too much.
• I've a descriptive title and added any useful information for the reviewer. Where appropriate, I've attached a screenshot and/or screencast (gif preferably).
• I've written tests to cover the new code and functionality included in this PR.
• I've read, agree to, and signed the Contributor License Agreement (CLA).

[codecov]

Codecov Report

Merging #173 into master will decrease coverage by 0.25%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
- Coverage   59.87%   59.62%   -0.26%     
==========================================
  Files           9        9              
  Lines         314      317       +3     
  Branches       57       58       +1     
==========================================
+ Hits          188      189       +1     
- Misses        112      113       +1     
- Partials       14       15       +1

Impacted Files	Coverage Δ
slackclient/user.py	68.18% <100%> (+1.51%)	⬆️
slackclient/server.py	58.77% <60%> (-0.92%)	⬇️

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[acaire]

@Roach, anything I can do to help move this along? :)