Add strava provider

[jstoup111]

@nraboy

I've added support for Strava. This flow only gets the code from Strava and gives you the capability to handle how you want the access token from there.

See #7

Retrieving the access_token is as simple POSTing per their documentation.

curl -X POST https://www.strava.com/oauth/token \
    -F client_id=5 \
    -F client_secret=7b2946535949ae70f015d696d8ac602830ece412 \
    -F code=75e251e3ff8fff

This is safer since the clientID wouldn't be stored client side.

[nraboy]

Your contribution didn't pass my tests. Can you please correct the issues and try again? Also, I only accept PR on the development branch. You'll have to clone the repository, switch branches, and apply your changes.

Best,