Add brushing to the legend to limit domains of mapped variables

[JackWilb]

Closes #184
Closes #186

In this PR, I'm adding brushing support for the legend charts. I've added support for glyphs (must be categorical), numeric node color, numeric link color, node size (must be numeric), link width (must be numeric). The chart brushing updates domains as necessary and is reflected in the visualization. There are a couple of other brushing interactions that I've pushed to issues for another PR -- I didn't want to make this too long. Once I've added the additional mappings, we could likely refactor this to reduce the duplication of some common logic (#248).

Many TODOS:

• Add brushing for bars (Add bar brushing support #245)
• Add brushing for glyphs
• Add brushing for node color
• Add brushing for node size
• Add brushing for link width
• Add brushing for link color
• Links now have opacity: 0.7;, this means that the color scales in the legend are now slightly off compared to the nodes. We can fix it by adding the same opacity to the scales in the legend
• Brushing chart extent doesn't match the width of the chart
• A group with a common name across glyph variables will be visualized if it only is brushed in one of the scale (bug! fix this in a follow up) (A label that is the same across the two glyph variables is mapped if it's brushed in one variable's chart #246)
• Fix categorical filtering for node color and link color (Fix categorical mapping for node color and link color #247)