Compimg

- Fixed documentation for `similarity` module (docs for metrics would
not appear)
- Python 3.8 officially supported (checking added to CI)
- Improve codebase by introducing black for formatting
- Added simple benchmarking so differences can be measured
when changes to existing code are made

- Improved performance of SSIM and GSSIM.
- Now using scipy to perform convolutions. Due to that now `compimg` is
dependent on `scipy`.
- Fixed issue where `_internals` package could not be found.

- Added `GSSIM` metric
- Added `RMSE` metric
- Added `MAE' metric
- Added `compimg.pads` module which provides easy way to apply padding to an
image (used in *SSIM implementations)
- Added`compimg.kernels` module which makes possible applying kernel to an
image (used within *SSIM implementations)
- More and better exceptions
- Moved `compimg.similarity.intermediate_type` to
`compimg.config.intermediate_dtype`
- Fixed `SSIM` metric (now implementation follows steps from the one
provided by authors)

This release fixes some small documentation errors, readme typos and
adds some badges to the README file. There are no actual code changes.