Add option to include a signature, conditionally.

[the-solipsist]

I've added an option to conditionally include a signature. I've changed the template, the yaml file, as well as the readme to provide instructions on how to use it.

[mrzool]

Not sure about this one… who puts a signature on an invoice? I never had to send out a signed invoice nor ever received one. Couldn't find a country where signing invoices is required by law. Seems unnecessary to be honest.

[the-solipsist]

In India, it is mandatory to have signed invoices (either through a pen-and-paper signature or through a digital (cryptographic) signature) for tax purposes, and there's software to help you do that as well.

[the-solipsist]

And further, if there's a closingnote, then it makes sense to allow for an optional signature even in contexts where a signature isn't legally mandated.

[mrzool]

My bad, looks like I didn't search hard enough. I'll have another look at this asap, reopening in the meantime.

[mrzool]

Ended up implementing this myself in 7569fde, without adding any additional option, consistent with how the same feature is implemented in letter-boilerplate.

Thanks for the input!