Plugin system shouldn't require a name

[bbatliner]

The new plugin vex-dialog-promise is a good use case for a plugin that doesn't need or want to add a new property to the vex namespace, e.g. vex.dialogPromise. This plugin is more transient in the sense that it modifies the existing vex API, in this case to promisify some of the methods.

A revision to the registerPlugin method will probably be necessary.