Add .gitattributes file

See wp-cli/wp-cli#5070

Update file(s) from wp-cli/.github

Merge pull request #194 from wp-cli/refine-feature-files

Merge pull request #189 from wp-cli/no-lock-file

Merge pull request #185 from wp-cli/revert-181-94-refactor-config-com…

…mand

Merge pull request #181 from UmeshSingla/94-refactor-config-command

Fix incorrect message shuffling salt in PHP 5.6 (#176)

* Fix duplicate warning

* Add feature test for duplicate warning when shuffling salts

Merge pull request #171 from wojtekn/add/mysql-sock-support

Merge pull request #166 from wp-cli/no-time-to-die

Merge pull request #164 from Ancocodet/fail-on-empty-table-prefix