set and honor mail-followup-to header
PEP8 fixes
docs: checkin autogenerated config docs
when determining the sender, also take From field into account
make reply code slightly more readable
Fix guess_encoding()

Turns out commit 7a9da3b0f obliterated a difference that mattered - the
different argument to magic.open() in guess_mimetype() and guess_encoding()

This commit reinstates that difference, and has a correct use of the pip
python-magic library to get the encoding.
Merge branch '0.3.4-fix-magic-again-602'
Merge branch '0.3.4-feature-mail-followup-to-599'
Fix duplicate display of headers

This separates the code dealing with
the case "display all headers" from the case
"display selected heders" in `MesageTree.construct_header_pile`
and ensures that 1. in "all in" case, no headers will be duplicated
and 2. in the filtered case, the order of headers given in
the whitelist parameter) is preserved.

fixes #601
Merge branch '0.3.4-fix-headers-601'