Support use of a custom line-item template instead of package default

[dshore]

Description

On occasions that require additional attribute detailing on the line item it would be nice to be able to specify a custom template.

Example use case of adding geoTargeting by modifying the line item template since it is not supported in the configuration; one could add the below as a custom template:

. . .

targeting:
  geoTargeting:
    excludedLocations:
      - id: 2840
        displayName: "United States"

. . .

AC

Add a CLI option to specify a custom line item template to use instead of the package default. E.g:

$ line_item_manager show template > exclude_US_template.yml
# edit exclude_US_template.yml adding above targeting to exclude the United States
$ line_item_manager create my_config.yml --single-order --template exclude_US_template.yml