[dev-context] Add toggle method to indent

Hi Hans, Can you add a `toggle` key to `\setupindenting`. This will allow one to easily write poety where alternative stanzas are to be indented. defineindentingmethod [toggle] {\ifdim\parindent=\zeropoint\relax \parindent\ctxparindent\relax \else \parindent\zeropoint\relax \fi} \setuplines [inbetween={\blank[preference,line]\setupindenting[toggle]}, indenting=no] \startlines .... \stoplines Thanks, Aditya