cleanup gruntfile

don't try to do gfm (github flavoured markdown):
the option in gruntfile seems to have no effect,
plus the gfm idea of adding breaks in html whenever
a break in input occurs does not help to maintain
readable input files.

don't try to share globals for various jshint targets:
the (detailed) target-level global will not merge with
the (more coarse grained) task-level global, but simply
replace it.