dictdiffer: exposure of `__version__` to clients

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
git: `dist` in gitignore

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
docs: fix for `revert()` example in Usage section

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>