Added jscallback method to Viewable objects

[philippjfr]

While jslink is very convenient for linking two objects it is also limited because it only exposes the source and target object, doesn't allow providing just a callback on the source and is not really designed around providing custom code.

This PR introduces a .jscallback method which can be used to add arbitrary JS callbacks, e.g. this example takes the values of the two slider and triggers an alert:

a = pn.widgets.Button(name='Add')
slider1 = pn.widgets.IntSlider()
slider2 = pn.widgets.IntSlider()

a.jscallback(clicks='alert(slider1.value+slider2.value)', args={'slider1': slider1, 'slider2': slider2})

pn.Row(a, slider1, slider2)

Cc: @xavArtley I know you were playing around with adding args to links but I think this generalizes this more.

• Clean up links.py
• Improve docstrings
• Write tests
• Write docs