Add "caves" command

[oubiwann]

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