Add Slack Provider

[hensur]

slack can return the user's team id and email address.

To restrict authentication to a slack team, I added the slack-team settings/cmdline option.

[martin-loetzsch]

This works quite well for us and as soon as there is an official fork (#628), then I'd love to see it integrated.

The older https://github.com/tappleby/slack_auth_proxy did not work for us, because it doesn't support the nginx auth_request directive.

This is how we install it: first install from master branch, then check out the PR and then install it again:

mkdir /path/to/oauth2_proxy
GOPATH=/path/to/oauth2_proxy go get github.com/bitly/oauth2_proxy
cd /path/to/oauth2_proxy/src/github.com/bitly/oauth2_proxy; git fetch origin pull/608/head:slack-auth-provider; git checkout slack-auth-provider
GOPATH=/path/to/oauth2_proxy go get github.com/bitly/oauth2_proxy

Thanks @hensur for creating this PR