Add ability to add collaborator

[mtscout6]

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[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@22b889c). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #462   +/-   ##
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  Coverage          ?   91.11%           
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  Files             ?       12           
  Lines             ?      405           
  Branches          ?        0           
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  Hits              ?      369           
  Misses            ?       36           
  Partials          ?        0

Impacted Files	Coverage Δ
lib/Team.js	100% <ø> (ø)
lib/Requestable.js	94.17% <100%> (ø)
lib/Repository.js	80.83% <100%> (ø)

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[MarkHerhold]

FYI, I opened PR #468 to to test against Node.js v8, which can be closed if this is merged.

[j-rewerts]

It seems like this API was considered preview? Newer docs have the URL being /repos/:owner/:repo/collaborators/:username, which is the same as the one that was used, so that's nice!

This PR would be easier to review if it didn't also switch the CI and add async await. @clayreimann Is there any plans to change which versions of Node this library builds for? I'm thinking we could merge in the addCollaborator stuff and leave async await for another day.