add version-strings into the module

The version string is in the module as __version__ available.
In the doc the version will be automatically parsed. The version comes
from the installed module if on read the docs or directly from
setuptools_scm if you are working locally.
The CLI will issue the version string if you call it with the option
'version'. The CLI does not accept commands like '--version', since this
would not be conform with the rest of the interface (and argparse).

The configuration for loading the version-string is adapted from
pimutils/vdirsyncer. It autogenerates a version string setuptools_scm at
install-time and then adds it to the __version__ member in __init__.py

I adapted the GitHub-template with a fitting comment and bumped the
changelog.

closes #141

CHANGELOG.rst

@@ -10,6 +10,7 @@ changes
 - packaging: configured the coverage-analysis codecov.io
 - GitHub: improved issues-template
 - documentation: add troubleshooting tip to network-interface
+- the module, cli and documentation is now aware of the version of python-escpos
 
 contributors
 ^^^^^^^^^^^^