Add :rest as a default symbol matcher
Add optimized is_get? method

The default get? method allocates a couple of strings, save those
allocations by using frozen string constants. As GET is the most
common request method, it makes sense to optimize for it.

This also allows is_get? to be overridden for different behavior,
which is dangerous to do with get? as that is defined the
superclass.
Optimize consuming patterns by using a positive lookahead assertion
Access env in request using instance/local variable

Instance variable access is faster than method calls, and
local variable access is faster than instance variable access.
Add head plugin, for handling HEAD requests like GET requests with an empty body

This isn't the default behavior as it slows things down and HEAD
requests aren't that common.
Update documentation for halt plugin
Minor plugin documentation tweaks