Added "Meetup.com" provider#220

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dance2die wants to merge 18 commits intoaspnet-contrib:devfrom

dance2die:meetup_test_branch

Commits on Feb 23, 2018

First working version of Meetup provider
dance2die
committed

Commits on Feb 24, 2018

Removed commented-out code and an unused using reference
dance2die
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Mapped all user's claim options
dance2die
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Added Meetup provider to the solution
dance2die
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Updated indentation as suggested by "kinosang" on https://github.com/aspnet-contrib/AspNet.Security.OAuth.Providers/pull/220#discussion_r170407336
dance2die
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Fixed indentation as suggested by kinosang on #220
dance2die
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Introduced a ClaimType static constant class "MeetupClaimTypes" as suggested by kinosang on https://github.com/aspnet-contrib/AspNet.Security.OAuth.Providers/pull/220#discussion_r170407367
dance2die
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Updated Meetup package author & company in the package (I have forgotten to update it)
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Commits on Feb 28, 2018