[FEATURE] Expose the "Bill to" string in invoice.go for customization

[lilith-writes-code]

Is your feature request related to a problem? Please describe. I'd like to change the "Bill to" string when using invoice.SetBuyerAddress(...) Currently it's a string buried in the implementation of invoice.generateInformationBlocks(...)

Describe the solution you'd like Any public method like, SetBuyerHeader or something that would let me set the string to be used.

[gunnsth]

Thanks for reporting, we will look into this as well as #332.

[gunnsth]

@venison please check PR #334 which addresses this.

[adrg]

Here's an example for setting the address heading:

invoice.SetBuyerAddress(&creator.InvoiceAddress{
    Heading:   "JANE L MILLER",
    Name:      "MILLER ASSOCIATES",
    Street:    "Suite #134569",
    Street2:   "1960 W CHELSEA AVE STE 2006R",
    City:      "ALLENTOWN",
    State:     "PA",
    Zip:       "18104",
    Separator: " ",
})

Also, added Street2 and State fields so it should address issue https://github.com/unidoc/unipdf/issues/332 as well. The Separator field is used to configure the separator between the address components (e.g. street, state, zip). By default, the separator is , in order to maintain backward compatibility.

Output invoice sample: invoice_simple.pdf.

[lilith-writes-code]

That looks great! Well done!