Show documentation when hovering over functions

[simonhorlick]

One key feature of the language server is to support contextual information via hovering over code. It's useful for developers to not only be able to see the types that a function deals with, but also it's documentation.

When hovering over String.flatten below:

      let str = String.flatten([err, "\n", hig]);

The user should be able to see something like:

String.flatten(xs: List(String)): String

flatten takes a List of Strings and concatenates them into a single String.

[simonhorlick]

This is partially implemented in #16, however documentation is not taken into account.