Imgwrench

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* dither subcommand for dithering
* filmstrip subcommand to stack images horizontally
* images can be saved in PNG format using :code:`--png` CLI flag

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* fix development status

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* option for preserving exif image metadata
* fix error when running with :code:`-k/--keep-names`
* status progress to Alpha

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* framecrop subcommand to crop and frame an image to a target aspect ratio incl. tests and docs
* breaking change: moved command modules to `commands` package
* introduced ImageInfo as a container for additional meta information in the pipeline
* increased test coverage
* more documentation

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* use a custom parameter type for colors

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* changed default frame width to 0.025
* usage doc for frame subcommand
* consistent alphabetic sorting of subcommands
* use a custom parameter type for ratios