Add .gitattributes file

See wp-cli/wp-cli#5070

Merge pull request #370 from wp-cli/fix-phpstan

Merge pull request #360 from wp-cli/fix/trunk-tests

Fix test involving WordPress trunk

Merge pull request #355 from wp-cli/fix/tests

Fix tests for Hello Dolly

Merge pull request #351 from wp-cli/refine-feature-files

Merge pull request #348 from wp-cli/update-phpunit-config

Merge pull request #335 from ernilambar/190-plugin-bootstrap

Merge pull request #326 from wp-cli/skip-mysql-tests

Merge pull request #324 from wp-cli/fix/wpcs-3.0-issues

Only install trunk on PHP 7.0+ (#321)