Fix badly chosen default positioning for sliders

[rreusser]

This is a trivial PR that fixes poorly chosen defaults for sliders. It should look nice by default, not by configuration. This PR sets:

{
  x: 0,
  y: 0,
  xanchor: 'left',
  yanchor: 'top',
  pad: {t: 20}
}

In many cases, this will hopefully just make it unnecessary to do much with the appearance. The result for an otherwise unstyled slider now looks like:

[etpinard]

thanks 💃