Ipn templates

[cmidgley]

Adds support for setting IPN based on Jinja2 templates. See #38 for background, and the updated README for feature details.

[30350n]

Okay, I quickly ran pre-commit on this to fix the formatting and also did some light refactoring:

• changed ipn_template to ipn_format (to make it more in line with the existing part_selection_format config var)
• changed --ipn options from "never", "new", "always" to "false", "true", "overwrite" (to make it more in line with the --interactive settings, functionality is the same repectively)

[30350n]

Okay, so some general thoughts on the implementation:

• Is it really necessary to use jinja2?
  I'd highly prefer an approach that just uses the standard str.format(...) function.
• It's nice that you already added extensive documentation for the feature, but I think it could be cut down a little bit. I'd prefer one or two sentences in the config.yaml section, with a simple example. (Let's do this last though, once we've settled on a final implementation).
• One thing I don't like is that this runs pre parameter-normalization. This is pretty my fault though, because it currently just gets done in setup_parameters(...). I'll refactor this on master by creating splitting this up into a match_parameters(...) function which adjusts the parameters on the api_part directly and will be run before committing the parameters to InvenTree. Then your import_part_ipn can be run inbetweeen those.