Add tinypng/tinyjpeg and/or KRAKEN.io online compressor via API

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for all of those who cannot install optipng and other necessary tools on their servers.

• Options/API Key could sit in a yaml config file.
• Tinypng's API reference: https://tinypng.com/developers/reference
• KrakenIO's API https://kraken.io/docs/integration-upload

Costs:

• TinyPNG: 500 API requests / month are free for everyone
• KrakenIO: MicroPlan is $5 / 500MB of images

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See #7