Add ability to use a different demo.json for development

[iturgeon]

This is being done in several widgets, let's move support into the MWDK.

const devDemo = 'demo_dev.json'
if (process.env.npm_lifecycle_script == 'webpack-dev-server') {
	options.demoPath = devDemo
}

• Add an option to getLegacyWidgetBuildConfig() named devDemoPath
• If devDemoPath set and webpack dev server in use, load the file from devDemoPath
• add a print output saying which demo.json is being loaded