Added SILC_STR_REGEX macro to SILC Buffer Format API.

	It provides powerful regular expression matching within the
	SilcBuffer.  Combined with SILC Buffer Format API it can turn
	the API into a Stream Editor (sed) and Awk like, providing
	powerful ways to match and edit the buffer in myriad of ways.
	The SILC_STR_REGEX can also mimic the behavior of sed (match
	and edit line by line).