Added support for '\x{...}

[cage2]

added support for specify a character using unrestricted hexadecimal number representation, for example:

"\\x{1F4E3}" → #\📣

also spaces between braces is supported:

"\\x{   1F4E3   }" → #\📣

This patch is a just a bit more than a POC, as I never touched cl-ppcre code before; but seems to works and I think it is a good start. Of course review or critics, suggestions are welcome!

Bye! C.