handle asset_host

Fixes https://github.com/Mange/roadie-rails/issues/10. Fixes https://github.com/Mange/roadie-rails/issues/11.

I was running into the problem as https://github.com/Mange/roadie-rails/issues/9, where I had stylesheets and images linked from my HTML emails. This PR takes a slightly different approach than the existing code, where it intercepts the `asset_host` as the email is being rendered, rather than using matchers to strip them after the fact, as was being discussed in https://github.com/Mange/roadie-rails/issues/10, which is messy for non-trivial `asset_host` Procs.

Please let me know if there are edge cases that I didn't consider!
Couldn't think of a good way to do integration tests for a string *and* a proc – open to ideas.