Allow selection of the ERB implementation.

This presents an avenue for configuration of the ERB implementation as
well.  It is an alternative to the following rejected pull request.
https://github.com/rails/rails/pull/7033

Specifically, one can subclass Erubis and include modules like
Erubis::PercentLineEnhancer to provide support for using '%' at the
beginning of a line to execute code without concatenating the result.

This should allow arbitrary ERB configuration without interpreting ERB
configuration inside actionpack.