When user runs unknown command, show the help

Closes #178

Also something to investigate would be to do a fuzzy search
for what the user intended to type and suggest that:

tj/commander.js#432 (comment)

bin/index.js

@@ -1,6 +1,9 @@
 /*eslint-disable */
 var program = require('commander')
 var packageJson = require('../package.json')
+var _ = require('lodash')
+var subCmd
+var cmds
 /*eslint-enable */
 
 program
@@ -9,3 +12,11 @@ program
   .command('build [directory]', 'Do a production build of site')
   .command('new [rootPath] [starter]', 'Create new Gatsby project in path [.].')
   .parse(process.argv)
+
+// If the user types an unknown sub-command, just display the help.
+subCmd = _.head(program.args)
+cmds = _.map(program.commands, '_name')
+
+if (!_.includes(cmds, subCmd)) {
+  program.help()
+}