Custom plugins does not overrides default commands

@artf , Until versions 0.9.x , GrapesJS loads all it's internal's modules before all the plugins are initialized. Since from version 0.10.2 you added a new module initialization after the plugins initialization. You can see it here: https://github.com/artf/grapesjs/blob/v0.10.2/src/grapesjs/index.js#L73 This behaviour has some side-effects, for example on grapesjs-preset-newsletter plugin because it tries to override the command 'export-template' but, when you initialized the modules after the plugins, this command is overridden again with the default command.