Add settings for multi-highlight

[gbarat87]

this close #3
Add Highlight + list of existing one

Settings for edit/new highlight with preview

[Fragonite]

Hi @gbarat87,

Thank you for working on this. I am still looking through the code but here is some initial feedback.

The live preview only updates on label change, it should also update when the colour is changed.

The enabled checkbox should be removed from the instance settings (keep it configurable from the filtersettingexternalcontent section).

Element order should be:

• Name
• Domains
• Label
• Text colour
• Background colour
• Preview
• Save changes/Cancel

Please use modern ES modules for all javascript and avoid using jQuery. The preview could be a mustache template with {{#js}} tags to load the module automatically when rendered.

Cheers.