New and optional fields with examples in README; various script improvements

[cseelye]

Handy project, I made a few changes if you are interested:

• Add additional fields COLOR2 and TYPE2, and make every field optional; empty fields will not be included in the sample
• Add explanation and examples of the fields to the README
• Skip any STL files that already exist
• Skip empty lines in the CSV file
• Add header row to the CSV file
• Parameterize input file and output dir as script arguments

[markusdd]

Hi! Sorry for the super late reply, I did not realize this PR was here, I completely overlooked it, must have missed the e-mail.

I like the changes a lot, but I have a local version already that also does the skipping so I need to compare that so we do not conflict, I then have to decide which one to keep. ALso, I would like a force option to decide to override nonetheless.

Regarding the header: I like the idea, but this is a backward-incompatible change. So could you add a little check that if this header is missing you just proceed with the first line as data?