Remove grunt from plugin scaffolding

[ernilambar]

Fixes https://github.com/wp-cli/scaffold-command/issues/140

• plugin-gruntfile.mustache is removed.
• plugin-packages.mustache is removed.
• plugin-composer.mustache is added.
• vendor/ is added in plugin-gitignore.mustache
• In composer wp-cli/i18n-command package is added. Script makepot is kept to generate POT file so that user can generate POT file like this command composer run makepot