Support for title templates

[pepicrft]

First of all, thanks for building this. I started using it for a new open-source project of mine, Gesttalt, and I came across the need for setting a title template that's used throughout all the routes where meta tags are set. Is this something you'd consider in the scope of the project? If so, is it ok if I go ahead with a code contribution in this PR? This is the API I was thinking about:

config :phoenix_meta_tags,
  titleTemplate: "%s - Gesttalt",
  description: "An open-source hub for social localization",
  url: "https://gesttalt.com",
  image: "https://gesttalt.com/images/squared-logo.jpg"

Where %s is replaced with the title of the route. I'm open to suggestions to make it more Elixirish.