`url_directory_name` is heavily called from any downloadable source
plugin, and moving it to cython gives a more than 10x speedup

Changes made to cli.py and _stream.py in order to support tar
compression. Compression flag has been added, which overrides any file
extensions given. Where no compression or file extension provided,
default to uncompressed .tar.

Avoid a restriction of Windows that prevents us from using both the file
descriptor and the file name of tempfile.NamedTemporaryFile.

Provide a wrapper that only returns the temporary filename, and makes it
easier to be windows-compatible.

Some plugin elements, such as docker-element and tar-element, require
to set `created` and/or `modified` meta-data fields. It would be nice
to use `magic_timestamp` to ensure these fields are being set in a
consistent and deterministic way.

This is needed in the case where the temp directory permissions
are changed between creation and deletion.

This is win32-friendly, less to read, and fixes a latent bug.

Avoid dealing with low-level platform specifics by using the
higher-level shutil.disk_usage() function to calc total and available
bytes for us.

shutil is also more correct - it uses f_frsize to convert from blocks to
bytes, instead of f_bsize. I verified that the numbers match with the
output from `df -k`, whereas the old implementation did not. Here are
the meanings from `man statvfs`:

    f_frsize    The size in bytes of the minimum unit of allocation on
                this file system. (This corresponds to the f_bsize
                member of struct statfs.)

    f_bsize     The preferred length of I/O requests for files on this
                file system. (Corresponds to the f_iosize member of
                struct statfs.)

Link to shutil.disk_usage() implementation:
https://github.com/python/cpython/blob/3.5/Lib/shutil.py#L980

copy_files and link_files suggested that a single file path is a valid argument
when it isn't.

This was discussed in #1008.

Fixes #1009.