make sure component attributes can be set on construction

[esheldon]

An example is label which can't be set on construction. This does not produce a visible PlotKey because label is not set

key=PlotKey(0.1, 0.9, halign='left')
plt=FramedPlot(key=key)
plt.add(Points([3,4], [5,6], label='stuff'))

[beckermr]

This will be easier if we address #82.

[beckermr]

Also #19

[esheldon]

looks like only style is being set from the keywords on construction

So we need to special case label (and maybe others) which is not a style

[esheldon]

The above PR put the framework in place for this, but so far I only found label for attributes to be set.