ENH: Improve styling of breadcrumbs

Tweak styling so that the breadcrumb navigation bar in an extension view
page does not look so out of place. In particular, set CSS 'overflow' on
the parent so that the bar is not painted outside of the parent element
(thus losing the nice rounded corners and producing a weird shadow) and
change the border color to match the parent's border color. Also, reduce
the margins a bit, giving the bar a slimmer appearance.
ENH: Allow suppressing breadcrumbs

Teach extension view page to honor a new query parameter 'breadcrumbs'
which, if set to 'none', hides the breadcrumb bar entirely. This is
desired for viewing the detail pages of installed extensions from within
Slicer, as it does not make sense to be able to navigate back to the
list of available extensions in this context.
Merge pull request #51 from mwoehlke-kitware/breadcrumb-tweaks

Breadcrumb tweaks