Support OSX #22
Description
When :SyncopateExportToClipboard is used on an OSX machine, it should invoke pbcopy instead of xclip. pbcopy doesn't seem to have HTML support (manpage), so for that to work, we'd need to send it RTF or PostScript.
:SyncopateExportToBrowser should also not default to sensible-browser on OSX, but that's not a big deal anyway since people can already configure the flag to whatever they want.