Add support for JavaScript file in HTML vis

[amir-zeldes]

@lgessler : the current architecture of the HTML visualizer relies on two files: NAME.config and NAME.css, as outlined in the ANNIS HTML visualizer guide. The idea is to add an optional third file, NAME.js, which allows the config to trigger JS functions from some common events defined in the config file (e.g. the onhover and onclick events of some of the elements generated by the .config file). Security/integrity of the rest of the GUI is a concern to keep in mind though.

The use case is to implement a signaling visualization on top of the existing HTML visualizer.

@thomaskrause : this is an old idea I wanted to revive, let us know if you have some ideas or concerns, esp. RE security issues.