Allow to use a pathlib.Path instance as a cd argument

[mariocesar]

from pathlib import Path

basedir = Path(__file__).parent.resolve()

@task
def deploy(ctx):
    with ctx.cd(basedir):
        ctx.run("ls -al")

Will fail with

File "/python/lib/python3.7/site-packages/invoke/context.py", line 94, in run
    return self._run(runner, command, **kwargs)
  File "/python/lib/python3.7/site-packages/invoke/context.py", line 100, in _run
    command = self._prefix_commands(command)
  File "/python/lib/python3.7/site-packages/invoke/context.py", line 242, in _prefix_commands
    current_directory = self.cwd
  File "/python/lib/python3.7/site-packages/invoke/config.py", line 123, in __getattr__
    raise AttributeError(err)
AttributeError: No attribute or config key found for 'cwd'

This is due that when getting the cwd value it expects to have an string type value.

By converting any cd argument to an string, it now supports to use pathlib.Path instances.

[russkel]

Relevant issue #454

[bitprophet]

Rolling into #607