add mailto helpers

parse_mailto parses mailto strings into headers,body and
mailto_to_envelope constructs an alot.db.envelope.Envelope
for a give mailto string
interpret 'compose mailto:..' from the command line
fir parse_mailto
docstring for mailto_to_envelope helper
Merge branch '0.3.5-feature-mailto-666' into testing
Added `Envelope-To` and `X-Envelope-To` in the search for my addresses in reply
Merge remote-tracking branch 'upstream/testing' into custom-path-completion-683