local: accept path-like (`pathlib.Path`) objects

[doronz88]

This will allow users to use the API as follows:

from plumbum import local
from pathlib import Path

local[Path('/path/to/binary')]()

[coveralls]

Coverage remained the same at 84.302% when pulling d25244e0bf8ff286ddf53a30b44c93e99f1ee872 on doronz88:master into a89f38169e0284ec99eeaaeace28fac3918dd479 on tomerfiliba:master.

[henryiii]

Thanks!