Compress-json

In this new version I have introduced:

- More tests, even the README is now tested as part of the test suite using [`pytest-readme`](https://github.com/boxed/pytest-readme)
- You can now specify the compression when it is not possible to infer from the file name with the `compression` parameter
- Handling of the `xz` extension

Cheers!

In this new version:

- I have added some small utilities and hand holdings, such as a more helpful exception for when it happens to swap the object and the path where the object needs to be stored
- Brought back test coverage to 100%
- Added more documentation