Usage information for each command with --help / -h

This is a proposal on how to have a consistent `--help` flag for each command. Similar to how `npm` does it, each command is a module that also exports a `usage` string key. This gets automatically printed on `dat <command> --help` (if available)

I also moved all double-commands like `dat blobs put` and `dat blobs get` to be submodules of `dat blobs`. For those cases the blobs.js module exports a function on `.usage` which gets the args as an input and can return the usage string for the appropriate subcommand.

So `dat rows` no longer shows `command not available` put instead prints usage informations instead.

Also the dat command itself shows this now:
```
$ dat
Usage: dat <command> [<args>]

where <command> is one of:
  cat
  export
  import
  init
  help
  version
  pull
  push
  clean
  clone
  serve
  listen
  blobs
  rows

Enter 'dat <command> -h' for usage information
For an introduction to dat see 'dat help'
```