Sever d3 dependency

[danvk]

d3 accounts for ~150k of the ~500k pileup bundle:

$ ls -lh node_modules/d3/d3*
-rw-r--r--  1 danvk  staff   329K Jul  3 23:00 node_modules/d3/d3.js
-rw-r--r--  1 danvk  staff   148K Jul  3 23:00 node_modules/d3/d3.min.js

Now that we're migrating to canvas (see #255), we barely use D3 at all.

Remaining uses:

• [x] d3.select(...).attr(...)
• [x] d3.scale (widely used)
• [x] d3.format (for scale & location tracks)
• [ ] d3.behavior.drag

Only the last of these will be tricky to replace. It would be nice if we could depend on only a small part of d3!

[danvk]

Some notes from doing a custom build of D3:

• scale.linear alone is 24k minified
• behavior.drag alone is 15k minified
• format alone is 12k minified
• all together they're 37k minified
• full d3 is 148k minified