Use escape_utils if available, fall-back on CGI.

[wpeterson]

• Try to load escape_utils and gracefully handle missing requirement
• Fall back on CGI.escape_html implementation
• Update README to explain how to use escape_utils

Benchmarks showing 10x-100x improvement.

[wpeterson]

I updated http://polarb.com/ to use the latest emoji gem without escape_utils and we saw a real performance hit. In a number of places we have 3 fields on a model we want web emoji support for and up to 10 models being rendered (30 emoji replacement calls in a given request).

I ended up doing a number of benchmarks and using escape_utils to replace HTML escaping for our entire Rails app, which cut our view rendering times about in half. So, this does seem to matter.

[steveklabnik]

👍