suggest additional command line option to accompany "--preview"

[csylvain]

trafficstars

suggest "--usewebkit" to enable use of the Qt webkit renderer when previewing ('viewer mode'). this option need not make the webkit renderer always enabled like it would be if the option is checked in the Edit menu.

the intended use case is component documentation written in markdown (module documentation) for a python app where it hands off the task of displaying the rendered markdown as a GUI