* Use os.PathLike in StaticFiles for directory
This allows using `pathlib.Path` in addition to `str` for configuring
the base directory of the static files in line with how python3.6+
handles filesystem operations.
Fixes#1004
* Fixed `mimetypes.guess_type` not supporting PathLike on py3.7 and below
* Updated staticfiles documentation with `PathLike` param
* I fixed a grammar error. The example code did not work, since there are no Path and PathPrefix classes in starlette.routing. I replaced PathPrefix with Mount to get it to work.
* I fixed a spelling error.