Add "title" to @type links

[eksperimental]

Add "title" to @type links. It's useful, so we avoid clicking, just the tooltip will come up showing us the values.

Examples where it will be used:

• http://elixir-lang.org/docs/stable/elixir/Dict.html
• http://elixir-lang.org/docs/stable/elixir/Agent.html#types_details

A solution like this one, just HTML5+CSS would be desirable:

• http://www.thewebmadeeasy.com/styling/create-html5-tooltip/
• http://www.table2css.com/create-html-tooltip-css-javascript

[eksperimental]

already implemented in ExDoc