Apply minor formatting and error capturing changes. Update README.

[EricCousineau-TRI]

I was able to get autocomplete working with the instructions in the updated README (and should hopefully address #1).

I also added some code to the parsing to (a) use argparse only and (b) by default compare to the merge-base between the destination branch (HEAD if none) and master.

@SeanCurtis-TRI Would you be able to give this a spin and see if it works for you?

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This change is 

[SeanCurtis-TRI]

This is excellent. Thanks for doing this.

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Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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drake_cloc.py, line 218 at r1 (raw file):

        src_name = "$({})".format(cmd)

    print("Computing difference between %s and %s.\n"

BTW You've upgraded this print statement to python 3, but not the previous. Any particular reason?

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Comments from Reviewable

[EricCousineau-TRI]

drake_cloc.py, line 218 at r1 (raw file):

Previously, SeanCurtis-TRI (Sean Curtis) wrote…

BTW You've upgraded this print statement to python 3, but not the previous. Any particular reason?

Er, not really. Just missed it - sorry about that!

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Comments from Reviewable