Sync up RegExPlus with functionality added in Java 7 / 8

For example, Pattern flag UNICODE_CHARACTER_CLASS

This allows \w and other built-in types to work with unicode characters. You can still pass the java.util.regex.Pattern int value, since RegExPlus uses Java's regex engine behind the scenes. However, adding this constant to RegExPlus directly will be much preferred.

Locate the RegExPlus entry point and its existing constants that wrap java.util.regex.Pattern flags. Compare them with Pattern.UNICODE_CHARACTER_CLASS, add the corresponding RegExPlus-facing constant, and verify that it enables Unicode behavior for \w and other built-in types.

Written by the indexing model from the issue text.