Token highlights should be done at mouse click

[AMecea]

Now the tokens are highlighted when the mouse is hovering a token, then when the mouse is moved the highlights change on another token. This can be annoying. I think it will be better to highlight tokens only on mouse click.

[chrismwendt]

I filed an issue about potentially adding this to the extension API https://github.com/sourcegraph/sourcegraph/issues/2064 Perhaps this functionality would be useful to other extensions as well.