New method regexp_with_modifiers()

Takes a single string of regexp modifiers and returns a regular
expression object.

Sneakily uses eval() to create the appropriate regular expression object
no matter whether we're on Perl before 5.14 or after.
New test: t/02-pod-example.t

This test covers the example from the SYNOPSIS section of Regexp::Trie
and ensures that it works as expected.