Add script to generate overrides CSS file

[kadamwhite]

This naively transforms the built CSS to the subset of rules for ONLY
look-and-feel styles; anything not relating to font or color is removed.

I am working on getting Webpack to respect the ordering so that this sheet
loads last.

Sample generated CSS output:

.topic-bar-chart .note {
  font-style: italic;
}

.topic-bar-chart .topic-bar-container {
  background-color: #bbb;
}

.topic-bar-chart .topic-bar {
  background-color: #838383;
}

.topic-bar-chart .bar-group.selected,
.topic-bar-chart .bar-group:focus,
.topic-bar-chart .bar-group:hover {
  outline-color: hsla(0,0%,60%,.1);
}

.topic-bar-chart .bar-group.selected .topic-bar,
.topic-bar-chart .bar-group:focus .topic-bar,
.topic-bar-chart .bar-group:hover .topic-bar {
  background-color: red;
}

We could make this smarter, or use it as a basis for a curated subset, or leave it as-is; thoughts?

[kadamwhite]

Note that when run from within the build script, this outputs the overrides file into both the public/ and the build/ directories, through the magic of that existing build script. We'll probably want to commit the local version in public/ so that we don't encounter a 404 on a missing stylesheet when checking out a new repo.

[kadamwhite]

Hold on merging, addressing issues with assistance from @mzgoddard

[iros]

@kadamwhite testing locally

[kadamwhite]

@iros test the generation and give me your thoughts on the output content, but this doesn't quite get injected right currently

[kadamwhite]

This now runs only on the production Webpack build, and injects the built file into the build/ directory as build/overrides.css.