Implement trigger keyword in commit message for pushing documentation to oso-api-docs

[Valefant]

With the issue #38, we have instructed Travis to push the backend generated documentation to the oso-api-docs repository.

The only problem now is that the commit history of the oso-api-docs will get bloated, because the documentation is pushed on every backend commit. To prevent this, we want to introduce a trigger keyword [generate api-doc] in the commit messages, which then allows to push to oso-api-docs.

Here are some hints on how to do this

• Outsource line 44-55 to a shell script called push-api-documentation.sh
• The environment variables which are defined in line 10-15 should be passed as arguments to the script
• Check at the beginning of the script if the string [generate api-doc] is included in the commit message. If it is then the commit to oso-api-docs should be pushed