Add the "open" component

[cpfannenmueller]

The open Component is missing and should be added, expeciallly when we can now place labels, as currently planed in the feature branch

\begin{circuitikz}[american voltages] \draw (0,1.5) -- ++(0.5,0)
       to[open, v=$v_o$, o-o] ++(2,0) -- ++(0.5,0);
    \draw (0,0) -- ++(0.5,0)
       to[open, v=$v_o$, voltage=straight, *-*] ++(2,0)
-- ++(0.5,0); \end{circuitikz}

[JPWiedemann]

This might be more complicated than initially anticipated since it behaves different to all the other path components. For now we don't support these exceptions. I have something in mind for the future though but it will take time and a lot more work on the component system