Add summarize CLI

[marcverhagen]

Pull out all code that is relevant to the summarizer proper in https://github.com/clamsproject/mmif-summarizer and add it here as a CLI tool

[marcverhagen]

Added working version in 0c40bac, but probably want to test that a bit more and do some other things including:

• Add some documentation.
• The script now takes -i and -o options for input and output filenames, consider doing the same as mmif describe, which allows standard in put and output.

[marcverhagen]

Would like to close this, but that is held up by failing tests for the pull request #350.

[marcverhagen]

Tests pass now. The summarizer needs to be extended for some kinds of input, but will leave that for a new issue.