Support `param.help()`

[jbednar]

HoloViews offers a help function hv.help(obj) that provides information on parameters and ranges for any Parameterized object, not just HoloViews objects. Can we move the generic Parameter-processing functionality into Param and offer param.help() to give information about any Parameterized object, extended separately by HoloViews but with the basics available to people who don't have HoloViews installed? Panel could then e.g. import help into its namespace for use as pn.help().

[maximlt]

Somewhat related to #425