Only intercept preflight requests for fonts, and bonus insert_target configuration option

[snikch]

Sorry this is a two in one

• Preflight requests are only intercepted for font assets. Makes the gem compatible with Rack::Cors or any other setup that also requires special preflight responses
• Added an insert_target option so that you can choose which middleware you want font_assets to be inserted before

[xevix]

+1

[ericallam]

Could you expand this PR to include some specs to demonstrate the features working? To run the specs all you have to do is bundle install and then bundle exec rspec spec

[sandeep45]

+1 Could use some merging

[ericallam]

Still looking for specs to prove this works before merging.

[glebm]

defined?(ActionDispatch::Static) never returns false, even if serve_static_assets is false. This should probably check for its presence in app.middleware instead.

[dmur]

+1, please merge -- this gem's current behavior of responding to any preflight request is really confusing.

[ericallam]

@dmur and @sandeep45, I can't merge this PR until there are specs proving this functionality works. It would be irresponsible of me to just merge in some code that is untested.

[dmur]

@ericallam Understood. I'm not a rails guy by trade or I'd write the specs myself.