Stegano

* new: Implemented LFSR generator (with tests and CLI)
([PR 27](https://github.com/cedricbonhomme/Stegano/pull/27))
* new: Implemented Ackermann generators CLI interface
([PR 26](https://github.com/cedricbonhomme/Stegano/pull/26))
* new: The Ackermann functions are not actual generators
([24](https://github.com/cedricbonhomme/Stegano/issues/24))
* new: add a shift parameter for the lsbmodule
([25](https://github.com/cedricbonhomme/Stegano/issues/25))
* fix: lsbset.hide cause .png transparent area lost
([23](https://github.com/cedricbonhomme/Stegano/issues/23))

* it is now possible to either pass the location of an image or directly pass
an already opened Image.Image to the hide and reveal methods;
* code re-formatted a bit with black.

* updated Pillow dependency to version 6.0.0 in order to fix a bug when opening
some PNG files (https://github.com/python-pillow/Pillow/issues/3557).

* updated Pillow dependency in order to fix a bug when opening some PNG files.

* added the possibility to shift the encoded bits when using the lsbset module.

* fixed a potential security issue related to CVE-2018-18074.