Elixir support (#48)

* Add elixir formatter

* Fix format for elixir to use stdout

* Add CHANGELOG entry

CHANGELOG.md

@@ -12,6 +12,7 @@ The format is based on [Keep a Changelog].
   for [LaTeX](https://www.latex-project.org/), and
   [rustfmt](https://github.com/rust-lang/rustfmt) for
   [Rust](https://www.rust-lang.org/) ([#24]).
+* [Mix format](https://hexdocs.pm/mix/1.12/Mix.Tasks.Format.html) for Elixir
 
 ### Bugs fixed
 * Fix spelling error in generated process names ([#32]).

apheleia.el

@@ -461,6 +461,7 @@ modified from what is written to disk, then don't do anything."
   '((black . ("black" "-"))
     (brittany . ("brittany" file))
     (clang-format . ("clang-format" file))
+    (mix-format . ("mix" "format" "-"))
     (gofmt . ("gofmt"))
     (google-java-format . ("google-java-format" file))
     (latexindent . ("latexindent" file))
@@ -489,6 +490,7 @@ commands, lists of strings and symbols, in the format of
   '((cc-mode . clang-format)
     (c-mode . clang-format)
     (css-mode . prettier)
+    (elixir-mode . mix-format)
     (go-mode . gofmt)
     (haskell-mode . brittany)
     (html-mode . prettier)