Color-matcher

This release contains the following changes:

- fixes to handle gray-scale and alpha-channel images
- refactored code for mkl_solver() method for better readability (comply with PEP8)
- added API function ColorMatcher.transfer(src=src, ref=ref, method='hm-mkl-hm')
- note that the existing ColorMatcher(src=src, ref=ref, method='hm-mkl-hm').main() is still viable

- new interface method 'mvgd' for analytical solution of a Multi-Variate Gaussian Distribution (MVGD) transfer
- move of the CI pipeline to [GitHub Actions](https://github.com/hahnec/color-matcher/actions)

This minor release update covers the following changes:
- output directory creation for processing a stack of input images
- CLI feedback about location of result(s)
- minor update on the [documentation](hahnec.github.io/color-matcher/)
- expand CLI unit tests (e.g. image dimensions)

- resolves image file batch processing from the command line following issue 1
- reduces repo size by removing redundant image
- set default paths for command line usage

The release v0.3.1 includes some minor updates and bug fixes including:
- added support for [Jupyter Demo using Binder](https://mybinder.org/v2/gh/hahnec/color-matcher/master?urlpath=lab)
- resolve uppercase letters provided in method string
- renaming of *./test* to *./tests* folder
- fix for TestPyPi upload in TravisCI
- removed .DS_Stores files

Release v0.3.0 features an analytical solution to the color transfer based on Multi-Variate-Gaussian Distribution (MVGD).