Add Segment.io provider so we can use it standalone

Since we'll be using our own fork of analytics.js with a custom
Dashboard.io provider, we won't be loading it from Segment.io's CDN.
But we actually want to use Segment.io too!

Their provider is actually only needed for a few additional services
that they support that aren't included in the open source repo, which
they provide some special server-side support for, like Salesforce. I'm
including it in our build for completeness, just in case.

I grabbed the provider code from the version of analytics.js served
from their CDN for our account. The provider itself is listed as MIT
licensed along with the rest of analytics.js; the script that it loads
remotely is copyrighted.
Put other providers we use back into our build