Add support for more compilers (#436)

* Add Mix compiler definition

This adds support for `:compiler mix` in Vim. The `'errorformat'` is set
to (partially) match output of `mix compile`.

TODO:

- resolve problem with setting correct paths for umbrella projects
- resolve problem with multiline errors

Issue #426

* Support for Credo as Vim compiler

Simple support for using Credo as a Vim compiler which provide simple
and easy way to lint code via `:make` command and QuickFix window.

Issue #426

Author: hauleth

compiler/credo.vim

@@ -0,0 +1,11 @@
+if exists('current_compiler')
+    finish
+endif
+let current_compiler = 'credo'
+
+if exists(":CompilerSet") != 2
+    command -nargs=* CompilerSet setlocal <args>
+endif
+
+CompilerSet errorformat=%f:%l:\ %t:\ %m
+CompilerSet makeprg=mix\ credo\ suggest\ --format=flycheck

compiler/mix.vim

@@ -0,0 +1,11 @@
+if exists('current_compiler')
+    finish
+endif
+let current_compiler = 'mix'
+
+if exists(":CompilerSet") != 2
+    command -nargs=* CompilerSet setlocal <args>
+endif
+
+CompilerSet errorformat=%A%t%*[^:]:\ %m,%C%f:%l:\ %m,%C%f:%l,%Z
+CompilerSet makeprg=mix