Regex::Iterable Fix

When using the constructor which takes a String rvalue, the Regex::Iterable's internal begin and end iterators were not being populated.
Regex::Iterable Begin/End String Iterator Accessors

Since Regex::Iterable is liable to create a new String internally (for normalization purposes), the begin and end iterators of that String are liable to change.

Accordingly, added a Begin and End method to the Regex::Iterable class so that consumers can access/use these iterators.
Callback Replacement

Added the Replace method to the Regex class, which replaces each match of the regex-in-question within a certain string with the result of a certain callback.