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 d25244e on doronz88:master into a89f381 on tomerfiliba:master.

[henryiii]

Thanks!