Override operator `+` for Path and its subclasses

[aleexf]

Operator + is not overloaded and comes from the base class str so it returns an instance of type str. This behaviour isn't obvious at all:

>>> type(plumbum.local.path(".") + "/subdir")
<class 'str'>
>>> type(plumbum.local.path(".") / "subdir")
<class 'plumbum.path.local.LocalPath'>

[zahlman]

For comparison, the standard library path manipulation:

>>> from pathlib import Path
>>> Path('.') + '/foo'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'PosixPath' and 'str'

Raising an exception makes more sense to me. Supporting the PathLike protocol is not the same thing as being a string subtype.