Provide Invoice2Data class for object oriented usage

[rmilecki]

This will allow using invoice2data in an object oriented way like:

import invoice2data

i2d = invoice2data.Invoice2Data()
i2d.extract_data("foo.pdf")

[rmilecki]

I was wondering if there is any interest in providing an Invoice2Data class so this project can be used as a library in an object oriented way.

That idea originally came from #392 but maybe it could be a generic improvement.

[m3nu]

Sounds good. So the class would be instantiated with some templates and then read invoices? Makes sense, since we already keep some state via templates and the input source.

[m3nu]

I'd still carefully consider which objects make sense. Maybe you also want an Invoice class to hold the read invoice.