Support output in "mutation elements" format

[ligurio]

There is a format of output for mutation tools called "mutation elements" [1].
Format supported by tools that used for visualization [2] and [3].
It would be nice to support this format in mutagen.

1. https://github.com/stryker-mutator/mutation-testing-elements
2. https://www.npmjs.com/package/mutation-testing-elements
3. https://github.com/ligurio/py-mutation-testing-elements