Use H for host option short flag. (#247)

Fixes subcommand help display (`-h`).
Add program help text to show subcommand help.

lib/bin/cli.js

@@ -22,7 +22,7 @@ program
 
 program.command('develop')
   .description('Start development server. Watches files and rebuilds and hot reloads if something changes') // eslint-disable-line max-len
-  .option('-h, --host <url>',
+  .option('-H, --host <url>',
           `Set host. Defaults to ${defaultHost}`,
           defaultHost
          )
@@ -57,7 +57,7 @@ program.command('build')
 
 program.command('serve-build')
   .description('Serve built site.')
-  .option('-h, --host <url>',
+  .option('-H, --host <url>',
           `Set host. Defaults to ${defaultHost}`,
           defaultHost
          )
@@ -80,6 +80,12 @@ program
     newCommand(rootPath, starter)
   })
 
+program.on('--help', () => {
+  console.log(`To show subcommand help:
+
+    gatsby [command] -h
+`)
+})
 
 // If the user types an unknown sub-command, just display the help.
 const subCmd = process.argv.slice(2, 3)[0]