Add "caves" command

Dev tasks:
* [ ] Add `.proto` dep on hexagram30/noise
* [ ] Add Go dep for the same
* [ ] Add new `commands/caves.go` file
* [ ] Provide flags for generating caves of the 6 supported types
* [ ] Provide flag for output type
* [ ] Provide flag for output file
* [ ] Provide flags for cave sizes (both pixel- and char-based)
* [ ] Update README with usage info/examples