Add CLI argument to get total/submodel parts count programatically

[nathaneltitane]

Is your feature request related to a problem? Please describe. Creating instructions with LPUb3D has proven somewhat unreliable as to how it handles and parses model/submodel info:

Automating the task of pulling relevant information from the model file directly using Leocad is less prone to have LPub3D mangle lines and introduce errors.

Having Leocad CLI be able to fetch model/submodel/parts information (and maybe other relevant model info such as weight/volume/overall dimensions) could prove handy when automating instructions header creation, as I am doing right now.

Describe the solution you'd like enable CLI argument for part count or other relevant info, i.e.:

$: ./leocad -parts ${model}

$: part_output