Forcefully convert unknown/custom instruction

A feature suggested by Dan Petrisko (https://groups.google.com/a/groups.riscv.org/g/isa-dev/c/TpMa23HZzXw/m/WsE1UtMbBAAJ) would be to let the user indicate an instruction type (e.g., R-type, I-type, etc.) to avoid failing for unknown instruction (which can be custom instructions). With an instruction type, we could at least show the binary fields.