Add support for RobotFramework .robot files

> RobotFramework is a generic test automation framework for acceptance testing and acceptance test-driven development (ATDD). It has easy-to-use tabular test data syntax and utilizes the keyword-driven testing approach. Its testing capabilities can be extended by test libraries implemented either with Python or Java, and users can create new keywords from existing ones using the same syntax that is used for creating test cases.

Its homepage is http://robotframework.org
It also has a GitHub Org at https://github.com/robotframework

Searching on Github can see many projects has adopted RobotFramework as a part of their testing solutions.
https://github.com/search?l=&p=1&q=extension%3Arobot&ref=advsearch&type=Code


PS. The RobotFramework also supports using .txt as a file extension for test case files, but I don't know how to make the [Bayesian classifier](https://github.com/github/linguist/blob/master/lib/linguist/classifier.rb) work in this case, so I am just adding support for .robot for now.

Thanks.