feat: pass additional data/vars into the templates

[davidalpert]

Sometimes when generating change notes it becomes useful to include data that is not present in the source code and is only known at build time while generating the change log. An example might be a build number that is generated by a CI/CD build server.

It would be nice to be able to pass variables into the changelog templates as command-line arguments:

1. a slice of string --arg name value flags similar to jq (e.g. --arg build_number 123)

   --arg name value
   

   This option passes a value to the jq program as a predefined variable. If you run jq with --arg foo bar, then $foo is available in the program and has the value "bar".

2. --arg-json '{"build_number": 123}'

3. --arg-file /path/to/args.json

   where /path/to/args.json looks like this

   {
     "build_number": 123
   }
   

in all cases the accumulated key/value pairs could be made available to the template as a map[string]interface and accessed via the index template function

{{ index .Args "build_number" }}

If this request is aligned with the direction of this project I would be happy to contribute a pull request.