Expose symlinks option for updates on macOS

[dennisvang]

On macOS, it may be necessary, as in #145, to enable symlinks when files are moved into the app_install_dir using shutil.copytree().

From the shutil.copytree docs:

If symlinks is true, symbolic links in the source tree are represented as symbolic links in the new tree and the metadata of the original links will be copied as far as the platform allows; if false or omitted, the contents and metadata of the linked files are copied to the new tree.

This PR exposes the symlinks argument so user can enable this behavior as follows:

client.download_and_apply_updates(..., symlinks=True)

If not specified, the default setting from shutil.copytree is used, i.e. symlinks=False.

fixes #145