feat: adds recommendCommands() for command suggestions

[bcoe]

working from @maxrimue's branch for recommendCommands(), and implementing @nexdrew's code-review.

This pull request gets us to v1 of command recommendations (thanks @maxrimue for the hard work) addressing some of the nits in #562 (@maxrimue sorry for @nexdrew and I providing such a difficult review).

Here's a breakdown of some of the changes made:

• rather than pulling in another dependency, I simply added a helper module for calculating levenshtein distance: this is a fairly simple algorithm, and I could imagine us using it in other places -- might as well dance around adding another dependency to yargs.
• we now handle the edge-case that @nexdrew outlined with nested arguments significantly better, by avoiding making recommendations until after we've walked through all the command handlers.
• we now recommend commands using the validation module, this allows us to piggyback off existing functionality in usage.js, and validation.js.
• we now use the same method signature as strict() for recommendCommands() rather than trying to be overly fancy with argument overriding.

reviewers: @maxrimue @nexdrew I plan on stubbornly marching us towards yargs@5.x over the next week, appreciate your patience.

[bcoe]

TODO: sort by length before applying algorithm, so that we take the least-changes/longest word first.

[nexdrew]

It looks like this is called even when argv._ only contains valid commands or arguments (called as part of running the command); it just potentially doesn't have any commands to match against.

[nexdrew]

The argv._.length check is actually redundant.

[nexdrew]

I would probably change lines 706 - 718 to this:

      // if there's a handler associated with a
      // command defer processing to it.
      var handlerKeys = command.getCommands()
      if (handlerKeys.length) {
        var firstUnknownCommand
        for (var i = 0, cmd; (cmd = argv._[i]) !== undefined; i++) {
          if (~handlerKeys.indexOf(cmd) && cmd !== completionCommand) {
            setPlaceholderKeys(argv)
            return command.runCommand(cmd, self, parsed)
          } else if (!firstUnknownCommand && cmd !== completionCommand) {
            firstUnknownCommand = cmd
          }
        }

        // recommend a command if recommendCommands() has
        // been enabled, and no commands were found to execute
        if (recommendCommands && firstUnknownCommand) {
          validation.recommendCommands(firstUnknownCommand, handlerKeys)
        }
      }

What do you think?