Apigentools

* [Added] Add command to merge spec files. See [235](https://github.com/DataDog/apigentools/pull/235).
* [Fixed] Fix detection of modified files for validation. See [234](https://github.com/DataDog/apigentools/pull/234).

* [Added] Add options `--force` and `--exit-code` to push command. See [228](https://github.com/DataDog/apigentools/pull/228).
* [Added] Bump openapi-generator. See [220](https://github.com/DataDog/apigentools/pull/220).
* [Fixed] Still write full spec in validate. See [232](https://github.com/DataDog/apigentools/pull/232).
* [Fixed] Allow empty GitHub information. See [227](https://github.com/DataDog/apigentools/pull/227).
* [Fixed] Fix no docker_run_options. See [223](https://github.com/DataDog/apigentools/pull/223).
* [Fixed] Fix parsing of APIGENTOOLS_API_VERSION env variable. See [221](https://github.com/DataDog/apigentools/pull/221).
* [Changed] Only validate changed files. See [231](https://github.com/DataDog/apigentools/pull/231).
* [Changed] Speep up write_full_spec YAML operations. See [230](https://github.com/DataDog/apigentools/pull/230).

* [Added] Add support for docker run options in test command. See [217](https://github.com/DataDog/apigentools/pull/217).
* [Added] Bump openapi-generator to latest master. See [212](https://github.com/DataDog/apigentools/pull/212).