Stegano

- it is now possible to specify the encoding (UTF-8 or UTF-32LE) of the message
to hide/reveal through the command line;
- the help of the command line now displays the available choices for the
arguments, if it is necessary (list of available encodings, list of available
generators);
- tests expected results lies now in a dedicated folder;
- a script has been added in order to get proper exit code check for mypy.

- updated command line. All commands are now prefixed with *stegano-*;
- improved type hints;
- it is possible to load and save images from and to file objects (BytesIO);
- improved checks when revealing a message with the lsbset module fails.

- improved generators for the lsb-set module;
- improved tests for the generators;
- improved type hints.

- unicode is now supported. By default UTF-8 encoding is used. UTF-32LE can also
be used to hide non-ASCII characters. UTF-8 (8 bits) is the default choice
since it is possible to hide longer messages with it.
- improved checks with type hints.

- introduces some type hints (PEP 484);
- more tests for the generators and for the tools module;
- updated descriptions of generators;
- fixed a bug with a generator that has been previously renamed.

- bugfix: fixed 12: Error when revealing a hidden binary file in an image.