Add easy way to list available plot methods and cleanup hvplot namespace

[philippjfr]

The hvplot namespace is polluted by param methods and attributes which I don't think are needed which means that tab-completing the plot methods doesn't work well. Additionally we should provide an easy way of listing the available plot methods.

[jbednar]

Will this be addressed by holoviz/param#230 once the move to the param namespace is complete? That's a good reason to try to hurry up that process...

[philippjfr]

Yes, but there really is not reason the HvPlot object has to be parameterized so there is an easier fix.

[philippjfr]

Namespace is now clean since we no longer subclass Parameterized.