Add dialyzer task to Elixir

Dialyzer is a discrepancy analyzer that ships as part of the Erlang VM. There are two projects that adds Dialyzer support to Elixir applications: dialyxir and dialyzex. The goal of this project is to bring the ideas of both projects with two main new features: better usability (in particular, better error messages and formatting) and the ability to dialyze projects incrementally.