Add cli client

[datitran]

Added a simple client to use our library in the terminal (refer to #45 for specs).

Now users can invoke our library like this:

imagededup find-duplicates --image_dir path/to/image/directory --method PHash

For more information on all available options, users can use the --help flag.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (dev@11b9999). Click here to learn what that means.
The diff coverage is 100%.

@@          Coverage Diff           @@
##             dev      #47   +/-   ##
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  Coverage       ?   95.46%           
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  Files          ?       17           
  Lines          ?      639           
  Branches       ?        0           
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  Hits           ?      610           
  Misses         ?       29           
  Partials       ?        0

Impacted Files	Coverage Δ
imagededup/client/client.py	95.23% <100%> (ø)

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[Stunkymonkey]

ping @idealo. I think more people are interested in having this.