Add ability to include aditional bom lines.

[Tyler-Ward]

This implements #51. An example of an additional bom line is shown below.

additional_bom_items:
  -
    description: Label, pinout information
    qty: 2
    unit: ea
    designators:
      - X2
      - X3
    manufacturer: generic company
    manufacturer_part_number: Label1
    internal_part_number: Label-ID-1

[17o2]

Thanks! Since the changes are very straightforward, I'll squash and merge if that's OK

[Tyler-Ward]

That sounds good to me