Add mocha as a test runner

[scottyantipa]

Setup mocha as a test runner and wrote some initial tests. This implements the same test suite as #49 which uses jest. This doesn't change the dist, and currently you need to manually invoke npm test (eg. no CI setup, and it doesn't run before a publish).

This PR adds:

• npm run test and npm run test -- --watch, npm run test:cover
• New libs: enzyme (react tests), jsdom (mock DOM), chai (assertions), and sinon (mocks)
• A new .nvmrc to pin a node version (v10 required for some of the test utils)
• Tests that get us up to about 70% coverage, still pretty minimal

So far the noticeable differences:

• jest is way easier to setup, batteries included
• mocha appears significantly faster, especially for --watch
• both offer similar functionality

Fix #51 see the issue for for more details on mocha vs jest