Add support for widget titles

[benlk]

This is kind of related to WPBuddy/largo#1201: This plugin doesn't output a title in the widget or provide a form for setting one in the backend.

• add a form and save function in the widget
• add title output to the widget

[benlk]

I'm not sure whether this should be in a subsequent release of the plugin or the initial release of the plugin. The initial release of the plugin should be feature-complete with Largo, but not having a widget title would be kind of weird for other sites. I'll ask the wp.org plugin review people about it.