new display format: plain text with clickable URLs

Hi,

I use MailCatcher quite a lot and often find it painful to copy-paste urls from plain text e-mails generated by the application I test, so I created a new view format - plain text, html-formatted in a very simple manner, with urls made to hyperlinks.
I'm not sure if the contributed code is good enough for the production version of MailCatcher (maybe a better name then "Less Plain Text" should be found for the new format; maybe should the conversion to html be done in a fancier way; ...) - take it simply as a proposal.