BREAKING CHANGE: `cli-plugin-unit-jest` and `cli-plugin-unit-mocha` now register
"test:unit" command and script instead of "test"; `cli-plugin-e2e-cypress` now
register "test:e2e" with optional `--headless` flag instead of "e2e" and
"e2e:open"; `cli-plugin-e2e-nightwatch` now register "test:e2e" instead of "e2e".
close#876, close#878
BREAKING CHANGE: PluginAPI.setMode() has been removed. Instead, for a plugin to
sepcify the default mode for a registered command, the plugins should expose
`module.exports.defaultModes` in the form of `{ [commandName]: mode }`.
close#959