Contributor Covenant: Update from v1.4 to v2.1

[justjwheelin]

This commit updates the Contributor Covenant used for this repository's Code of Conduct from v1.4 to v2.0. The most significant change between the versions is the addition of a consequence ladder, or a way to transparently share moderation guidelines and principles through the Code of Conduct.

Although I hope to not have to use this, I believe the v2.0 changes are helpful in providing structure and process in small communities like ours where we have not fully explored what equitable consequences and moderation best practices are.

CC: @jancborchardt @sleepypioneer @jonatoni @kristiprogri

[jonatoni]

@jancborchardt @kristiprogri can you please review/approve this PR?

[justjwheelin]

I updated the pull request to include the latest v2.1 version of the Contributor Covenant, which includes very minor language additions from v2.0.