Strategy on overriding certain options, storing default options

[formatc1702]

#214 has added the possibility of configuring certain options (colors, fonts, BOM formatting, language for color display names, ...) in the YAML, as originally proposed in #158.

The following functionalities would be highly desirable:

• To be able to define "global" defaults, to avoid defining them in every individual harness YAML file.
• To be able to override options from the command line for one-off use
  (e.g. quickly export a version with German color names on the fly, without touching the YAML)

A hierarchy of prioritization of the different configuration sources should be defined, for predictable outputs.
Proposal:

1. Setting options via command line parameters (not likely to be required often, but should have # 1 priority)
2. Setting options in the harness YAML file
3. Setting options in the prepended YAML file (likely to be used for user- / company-wide defaults to use in all harnesses)
4. Setting options in the WireViz config file* (currently not implemented).

*Some ideas on the config file:

• Could be a YAML-formatted file, perhaps named .wvconfig or similar, and stored in a user's home folder
• If it exists, it should set the default options to use whenever running wireviz
• Could be used to specify a default prepend file, to avoid the need for specifying it in the CLI every time
• Could be expanded over time to cover many more config options
  • Which columns to display in the BOM table, in what order, which language to use for column headers, ...
  • ...

[kvid]

I also suggest a CLI option to ignore any global options. This is useful when wanting an output that only depends on the imput YAML file, e.g. when running build_examples.py for comparison purposes.