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).