Add tests

[nickserv]

Fixes #53

• Expose code for testing
• Test basic button injection
• Create fully-functioning stubs
  • chrome.extension.storage.sync (needs to be custom)
  • MutationObserver
• Attempt to test with mocked window.location
• Test option syncing behavior
• Scope ESLint config for tests
• Consider adding more granular unit tests per class/file/method/function
• Complete test coverage

[kaicataldo]

This is great! I would advocate for getting the testing infrastructure set up in this PR and landing it so that it frees other people up to start writing tests. What do you think?

[nickserv]

This got a bit messy with some unrelated test code and configs being combined, so I'm going to split it up into multiple PRs.