Allow user to enable/disable the plugin in Mirador's config

[D-Groenewegen]

Currently, there is no configuration option to let you either enable or disable the plugin. When you've imported it for a module bundler like Webpack, it must be configured and made available. It would be more user-friendly if the option could be set in Mirador's config object. The alternative is to create two separate builds of Mirador, one with and the other without the plugin, which is a little cumbersome.

Suggestion

• Add the config option enabled under annotations, much as has been done for other plugins like CanvasLink and TextOverlay
• Make it a boolean. Default value: true.
• Possibly, make it a window-specific option.

P.S. I did notice that the ImageTools plugin does things a little differently. Maybe it's up to the Mirador developers to standardise the way a plugin can be switched on/off.

[D-Groenewegen]

Just a minor update. While I still think this would be a useful option, I have now solved things differently.