Highlight TOC with `current`

[ericholscher]

Currently you can't see where you are in the TOC because the current TOC entry isn't styled. It should be at least made bold or similar.

We have a bunch of styles around this for the RTD TOC, but at least doing a basic addition like this would be useful:

https://github.com/rtfd/sphinx_rtd_theme/blob/505fbb5f7e90a9e65bad5eca4f502c269bcf5796/sass/_theme_layout.sass#L48-L55