cmd/cue: add "generate" command

[cueckoo]

Originally opened by @mpvl in cuelang/cue#26

Analogous to the Go's go generate.

CUE's is designed with code generation in mind and it is to be expected that many CUE files will be generated. A cue:generate directive would allow annotating the methods used to generate the files in a way familiar to Go programmers.

[cueckoo]

Original reply by @mpvl in cuelang/cue#26 (comment)

The obvious thing to do is to add a command: generate: entry in the tool layer. Until this is proven to be awkward, we archive this option.