Improved nimdoc styling.

This updates the inlined CSS in nimdoc.cfg, adds several helper classes
(and a few divs) to the generated HTML (some in nimdoc.cfg, one in
rstgen.nim), and tweaks the anchor linking scheme for headers in
rstgen.nim.
Update nimweb.nim to work w/ non-hardcoded --docSeeSrcUrl argument to nim doc
Merge pull request #1748 from boydgreenfield/improved-doc-styling

Improved doc styling