Improve output of diagram + BOM as technical drawing

[formatc1702]

New version of #74.

Reason for new PR:

• 74 was based on the original branch for #214, called feature/metadata
• # 214 was then squash-merged onto dev.
• Deleting feature/metadata (since it was no longer needed for # 214) without changing # 74's base first, cause it to auto-close, with no way to re-open

[formatc1702]

This PR is ready for review. I have been using this feature and the included DIN 6771 template files for quite a while and I am very pleased with the results.

In the interest of not delaying the merge too much, I would request to address the following minor issues as separate PRs:

• Optimization of the HTML templates themselves
• Dealing with resolving template file paths specified in the prepend YAML file.
  • I would like to fix this together with the related issue from the comment in #189 afterwards
  • The newly added smart_file_resolve() may help fix both issues
• Parsing any color/font specs from the options section of the YAML and applying them to the HTML
• I have purposely only edited demo02.yml to include metadata and template specs, but not demo01.yml, so as to keep 01 as simple as possible.

[formatc1702]

• This preliminary review is only based on inspection of changes, not testing.

Thanks, I will incorporate some of your changes soon and wait for some testing.

• Consider also replacing all keywords from the template in one pass to avoid replacement of replacements (and to increase efficiency), e.g. like in the lead answer here: https://stackoverflow.com/questions/6116978/how-to-replace-multiple-substrings-of-a-string

👍

• Have you considered supporting an optional default value of each keyword in templates? (e.g. something like this: <!-- %bgcolor%white% -->)

Sounds like a nice follow-up PR.