* add issue_invoice_tx command
* rustfmt
* add first pass at process_invoice command
* start of process_invoice fn
* rustfmt
* rename issue invoice and process invoice to invoice and pay
* add prompting and display information to pay invoice command
* rustfmt
* support invoice transactions in finalize command
* rustfmt
* add slate upgrading/downgrading
* add slate versions
* slate versioning compilation and tests
* transaction slate responses downgrade themselves to the version of the caller
* add command line arg for target slate version output
* v2 versioning comment info
* doctest fixes