Compatibility and customizability improved

• Moves development instructions to CONTRIBUTING.md, which will appear in Github UI when submitting new issues and PRs, including which branch to use for 2.x and 3.x
• #280 - Restore targetSelector option in test helpers
• #282 - Add support for popper.js position variants (e.g. `
• #285 - Add ability to override options passed through to popper instance
• #283 - Use ember-get-config instead of this.getConfig in order to support Ember engines
• #286 - Add ember-line-clamp compatibility coverage
• #289 - Add Ember Fastboot support and coverage
• #290 - Prevent tooltip from adding target element's title attribute to tooltip
• #291 - Fixes {{ember-tooltip}} typo in Readme