Optionally reply below quote (email style)

This is an extended version of the patch submitted already in https://github.com/Bitmessage/PyBitmessage/pull/271, see also https://bitmessage.org/forum/index.php?topic=3644.0.  This version implements a new preference (which is off by default) and only uses the new quoting style if the user actively sets the preference to on (opt-in).