Add %(unmailto) format function for List-Post headers

With the unmailto format function it is possible to extract
the address from a List-Post header, which e.g. is:
	List-Post: <mailto:mmh@marmaro.de>
Applying  %(unmailto{list-post})  extracts: ``mmh@marmaro.de'',
which then can be used as recipient address for list replies.
A format file (to be used with ``repl -form repllistcomps'')
is included.