Stegano

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

* Fixed an encoding problem which occured on Windows during the installation
of the module.

* Stegano is ready for use with pipenv and pipsi.

* the recommended way to install Stegano is now to use pipenv.

* Fixed a bug with the new 'encoding' function when using Stegano as a command
line tool. No default value was set. Default value is UTF-8.

* 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.