Only add :elixir application for Elixir projects in compile.app task
Support building Erlang-based escripts in mix escript.build
Document the :language setting
Add a small note to UndefinedFunctionError when the module is not available
Add Task.start/1 and Task.start/3
Use task start to get better reports
Fix the build
Use common code for Elixir and Erlang escripts
Merge pull request #2687 from alco/mix-language-setting

Mix language setting