Official API for capturing diff output

[timothycrosley]

Capturing diff output is a common need for sub projects that depend on isort. Currently, there is no official API method of doing so resulting in downstream users needing to use creative and undocumented solutions to capture the output. The proposed resolution is:

• Make sure show_diff is available as an argument for all API methods.
• Allow either a boolean or TextStream to be provided. If a text stream is provided it should be used, otherwise if True is provided sys.stdout or the current output stream should be used.