broken links / RFC 2646 format=flowed support

[jkbzh]

Messages may be sent in a format called format=flowed where the lines are wrapped in a special way that lets them be reliably unwrapped later. Hypermail doesn't handle this format. This results in broken links when a paragraph is flowed and the link is broken out between two lines. If format=flowed were supported, the lines would be unwrapped before applying the link detection code.