Make the SMB plain-text only feature available to users of the

MsgBase.get_msg_body() method: added an additional boolean argument (following
the 'include_tails' argument), which defaults to false (disabled). Pass true
for this argument to enable this feature. Yes, JS-authors could do this parsing
easily via JS, but I figure so long as this feature is in the SMBLIB, might as
well expose it to JS for consistency (e.g. in 3rd party message reader mods).