Add cost and api cost options

[alpe]

Resolves #525

I have started with public vars but stopped as this would open the door for concurrent access from outside the module. As a library we should protect against that. A simple but safe alternative are constructor Options. It was not in my original lists of proposals though.

[ethanfrey]

Looks like a good approach to allow this to be configured in app.go without adding runtime overhead (db reads for params approach)