Add headerHeight and options to lrStickyHeader
Add headerHeight and options to lrStickyHeader

This change allows users to specify a header height in pixels. This will
be used by the sticky header to make the header stick at an offset.
Particularly useful if you have a bootstrap nev bar with
navbar-fixed-top set.

lorenzofox3/lrStickyHeader,lorenzofox3/lrStickyHeader