Elixir plugin for JetBrain's IntelliJ

The plugin works both in the rich IDEs that allow alternative language SDK selection and small IDEs that are language specific. The rich IDEs work best for IntelliJ Elixir because only in the rich IDEs can have an Elixir SDK set as the Project SDK. In all small IDEs, the native language SDK is always there, which makes anything that uses the SDK, such as running elixir, erl, or mix more complicated both internally and externally in the configuration you have to setup. The plugin will automatically find the newest version of Elixir installed. (NOTE: SDK detection only works for Linux, homebrew installs on OSX, and Windows. Open an issue with information about Elixir install locations on your operating system and package manager to have SDK detection added for it.) If the automatic detection doesn't find your Elixir SDK or you want to use an older version, manually select select the directory above the bin directory.