`cargo install` or some way to install the man page

I had to manually run `install -m 644 rgbobj.1 /usr/local/share/man/man1`.

An rgbds-style Makefile could suffice:

```make
.PHONY: install

Q       := @
PREFIX  := /usr/local
mandir  := ${PREFIX}/share/man
MANMODE := 644

install:
	$Qcargo install
	$Qinstall -m ${MANMODE} rgbobj.1 ${DESTDIR}${mandir}/man1/
```