Doc impr: auto generate url to Python's website for lib API

[litlighilit]

original draft:

when defined(nimdoc):
  macro pydoc*(decl) =
    let symId = _getName decl
    let sym = symId.strVal
    let moduleName = _getModuleName()
    let url = "https://docs.python.org/<PyMajor>.<PyMinor>/library/" & moduleName & '#' & moduleName & '.' & sym
    var nDecl = decl
    nDecl._addDocUrl url
    result = nDecl
else:
  template pydoc*(decl) = decl

Note: symbols that begins with underscore is seudo operator (not impl here).

[litlighilit]

Also, maybe helps to support pydoc.help