Handle extra parameters

[Mirch]

The build command should be able to take in extra parameters that match placeholders in the pont.yaml file. When running the build command, these the placeholders should be replaced before running the commands.

Example:

pont build --name foo --from https://github.com/foo/bar baz=fred

version: 0.2.0
name: foo 
commands:
  - echo "{{baz}}"

The output of running the build command in this case should be:

fred