closes: #402
closes: #442

Allow user to exclude some parameters.
Fixes https://github.com/lesspass/lesspass/issues/528

Try idea from #60

* use `choices` attribute from parser.add_argument() instead of complex actions override

* use metavar to improve --help output

* use custom 'range_type' for length argument

Fixes https://github.com/lesspass/lesspass/issues/408

* test _calc_entropy()

* test _get_configured_rules()

* test _get_set_of_characters()

* test _consume_entropy()

* test _get_one_char_per_rule()

* test _insert_string_pseudo_randomly()

* test _render_password()

create a cli with python.
python will be installed by default on a lot of system.

 * use getpass python native module
   Fixes: https://github.com/lesspass/lesspass/issues/305

 * use lesspass python code generator from Maurits van der Schee
   Fixes: https://github.com/lesspass/lesspass/issues/328

tested with python2.7 and python3.6

breaking changes:

 * -c is associated with --copy flag to copy the password
 * -C is associated with the counter (previously -c)