extend the link_regex to allow mailto: links

At the moment only http/https/ftp links are allowed when creating a link in the text-block. This extends the regex used to test the link to allow mailto: links. 

Accepts the following:
ftp://255.255.255.255 true 
http://www.domain.com true 
http://domain.com true 
http://domain.co.uk true 
https://www.domain.com true 
https://domain.com true 
mailto:user@domain.com true 
mailto:user.name@domain.com true 
http://localhost true 

Does not accept the following:
user@domain.com/dir?param=value false 
256.256.256.256 false 
ftps://sub.domain.com/.ext false 
domain.com/?param=value false 
domain.com/dir/page.ext?param=value&param=value false 
mailto://asdasd@asdasd.com false 
mailto:asdasdasdasd false 
mailto://asdasd.com false