Expand the scope of the syntax highlighting to add more color schemes

[georgecarstoiu]

Minor enhancement request: expand the scope of the syntax highlighting to add color schemes to:

• predefined keywords (BuiltIn, String, SSHLibrary etc)
• user defined keywords
• keyword args (id, xpath, name etc) - not their values

[aaltat]

I know that there are people that like their color schemes to be bright and have many different colours. I am in the opposite side and like on the minimalist style. That being said, I am happy to review a pull request to enhance the syntax highlighting. The current highlight definition can be found from: https://github.com/andriyko/sublime-robot-framework-assistant/blob/master/Robot.YAML-tmLanguage file.