Fix configure issue

[ashkan18]

Problem

Configuring artsy_api_url in OmniAuth::Artsy doesn't change site in OmniAuth::Strategies::Artsy.

Solution

This is because of the fact that option method in Strategies::Artsy was called at load time and when we configure OmniAuth::Artsy it's too late and doesn't impact how strategy was setup. Not sure what's the best solution here, but I ended up:

1. Move option call in OmniAuth::Strategies::Artsy to configure method and call it once during load time for existing users of this gem.
2. Add reconfigure_strategy method to OmniAuth::Artsy which will be called after running configure block and will call configure method mentioned ☝️