Fix #117 - Add command to open or create tag page

[mvaaltola]

Add command to open a tag page or create one if it doesn't exist. Also handles internal and external links so it can be used to replace the default "Follow link under cursor" command (Alt+Enter by default).

Closes #117.

[pjeby]

I think what I actually want to do is provide settings in the plugin for whether you want to have tag clicks (and Follow link under cursor) open tag pages or search, and then patch Editor.prototype.triggerClickableToken() to behave accordingly. In this way, no new commands or keybindings are required, and I want to provide settings for this to address other outstanding issues regarding what happens when you click tags in other contexts.

That's going to require some careful consideration, though, as it's not just a matter of one toggle, as I want to have options for how it will respond to other kinds of clicks, too.