Add support for changing options dynamically.

Motivation: to make this plugin more usable for pages that use responsive
Bootstrap styling. On such pages, it is common for high-resolution views
to use a floating "sticky" nav bar (in which case StickyTableHeaders
needs an offset that's >= 0), while lower-resolution views might make
the nav bar "non-sticky" to conserve screen space. When the viewport
changes, it is necessary to tell StickyTableHeaders to change the
fixedOffset option so that it can account for the varying "stickiness"
of the nav bar. Prior to this commit, the only way to accomplish this
was to destroy the StickyTableHeaders instance and recreate it. I think
this fix is much easier and more efficient!