Add support for arbitrary/custom fields in part description

[psychogenic]

Greetings,
I wanted a way to easily generate symbols for part--which kipart is great at--while specifying certain extra fields for spice simulation.

I've pushed this patch to enable this functionality, while keeping it generic and general purpose. In short, this version supports a 7th field that can hold any number of entries for the part, in the form
A=B
VendorPN=1234ABC

etc. Which creates appropriate F* entries in the generated lib.

I've included an example-with-customfields.csv. Hopefully this will be useful to you and others as well.