Add possibility to use additional babel plugins

[jefersonla]

Add additional babel plugins to the babel config, loading the configurations from package.json file.
To add additional plugins just add a property babel_plugins to enact property in package.json, in the same way plugins are defined on babel config files.

package.json example:

{
	...
	"enact": {
		"theme": "moonstone",
		"babel_plugins": [
			[
				"@babel/plugin-proposal-decorators",
				{
					"legacy": true
				}
			]
		]
	},
	...
}

[CLAassistant]

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[jefersonla]

Sorry for the multiple submissions. Prettier for some reason is not working in my computer so I've used travis log to format properly my code.