Root path option

I wasn't able to get the tabs to work with existing routes and because the window location would be changed from the base, e.g., `cheese/cheedar-tab` would be directed to `cheddar-tab`.

I may have misunderstood how to use the package though, so apologies if it is already possible.

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Optionally give a root path onto which the tab paths will be added.

This allows you to have tabs at the end of paths e.g.

    /myapp/tabname
    /myapp/anotherTabname