allow to set datetime format in format_date and format_time
allow to set datetime format in format_date and format_time

This is especially usefull with the generic formats 'long',
'medium' and 'short'

liqd/adhocracy,alkadis/vcv,phihag/adhocracy,alkadis/vcv,liqd/adhocracy,phihag/adhocracy,DanielNeugebauer/adhocracy,alkadis/vcv,DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,liqd/adhocracy,phihag/adhocracy,phihag/adhocracy,phihag/adhocracy,DanielNeugebauer/adhocracy,liqd/adhocracy,DanielNeugebauer/adhocracy,alkadis/vcv,alkadis/vcv