Add customizable Jinja context configuration option

[bkryza]

For configuration files with large number of diagrams which rely on Jinja templates, it would be useful to add context option to the inheritable config option set, which would enable for instance the following use case:

...
context:
  author: bkryza
  copyright: Copyright (C) 2023
  my_condition_1: true
...
diagrams:
  my_class_diagram:
    type: class
    ...
    plantuml:
      after:
        - >
          note left of {{ alias("ClassA") }}
             {{ author }} {{ copyright }}
          end note
        - >
          {% if my_condition_1 %}
          note left of {{ alias("ClassB") }}
             {{ author }} {{ copyright }}
          end note
          {% endif %}

There should be also a command line option to override/add any context variable, e.g.:

clang-uml -n my_class_diagram --context "my_condition_1: false"