Specify requisite identifiers on plugins

[joel99]

Plugins that use individual identifiers (e.g. email, sms) should specify so with a target() function and a needs attribute that specifies which identifier they want (e.g. email addresses, phone numbers) as a string. target will take an array of needed identifiers provided by the server and adjust plugin config accordingly. When sendMessage is called on the plugin, it should check if config has target info, and dispatch notification to all targets, otherwise have default behavior. Fail appropriately if target is called with invalid values.

Part of #13 epic.