Add auto-increment support to version command

Pass "major", "minor", or "patch" to increment the existing version
by that amount.

doc/cli/version.md

@@ -10,8 +10,13 @@ npm-version(1) -- Bump a package version
 Run this in a package directory to bump the version and write the new
 data back to the package.json file.
 
+The `newversion` argument should be a valid semver string, *or* a valid
+second argument to semver.inc (one of "patch", "minor", or "major"). In 
+the second case, the existing version will be incremented by that amount.
+
 If run in a git repo, it will also create a version commit and tag, and
 fail if the repo is not clean.
+
 If supplied with `--message` (shorthand: `-m`) command line option, npm
 will use it as a commit message when creating a version commit.
 

lib/version.js

@@ -21,10 +21,11 @@ version.usage = "npm version <newversion> [--message commit-message]"
 
 function version (args, cb) {
   if (args.length !== 1) return cb(version.usage)
-  var newVer = semver.valid(args[0])
-  if (!newVer) return cb(version.usage)
   readJson(path.join(process.cwd(), "package.json"), function (er, data) {
     if (er) return log.er(cb, "No package.json found")(er)
+		var newVer = semver.valid(args[0])
+		if (!newVer) newVer = semver.inc(data.version, args[0])
+		if (!newVer) return cb(version.usage)
     if (data.version === newVer) return cb(new Error("Version not changed"))
     data.version = newVer
     Object.keys(data).forEach(function (k) {