Add :escape option to render plugin for using erb templates where <%= %> escapes and <%== %> does not

Erubis has native support for escaping by default, but unfortunately
it is broken if you use postfix conditions inside <%= %>.  Add our
own Erubis::EscapedEruby subclass working around that issue.  While
there, optimize the generated template by using a local variable
instead of a nested constant lookup for each escape.
Add :content option to view method in render plugin to use given content instead of rendering a template

When using :escape, you can't use an inline template with <%= content %>
to render arbitrary content inside the default layout.  This lets
you provide the content to use if doesn't require a template rendering.
Enhance security section in README