Added support for links with target="_blank" when NOT matching PCRE for internal links

Generally it would be nice, if the parser could recognise if a link is referring the internal site, or something external, in which case the link should open in a new window.
This patch implements the possibility to specify a PCRE in a parser instance (public $internal_regex) used for matching internal links. If a PCRE is specified than every link is checked against it. If it does not match, than a "target='_blank'" is added to the link markup.

Example:
Using the expression "#(^(http|https)://www\.yeasoft\.(de|com|net|org).*$|^/.*$|^((?!http://).)*$)#" the following URLs are matched as internal:

http://www.yeasoft.com
https://www.yeasoft.net/projects:alix
/site/project:btsync-deb
relpath/xyzzy.html

The following URLs are recognised as external:
http://github.com/tuxpoldo
http://www.google.com