Add defn syntax highlighting

Fixes #550.

Author: duff

syntax/elixir.vim

@@ -145,6 +145,7 @@ syn region elixirDocString matchgroup=elixirDocSigilDelimiter  start=+\%(@\w*doc
 " Defines
 syn match elixirDefine              '\<def\>\(:\)\@!'             nextgroup=elixirFunctionDeclaration        skipwhite skipnl
 syn match elixirPrivateDefine       '\<defp\>\(:\)\@!'            nextgroup=elixirPrivateFunctionDeclaration skipwhite skipnl
+syn match elixirNumericalDefine     '\<defn\>\(:\)\@!'            nextgroup=elixirFunctionDeclaration        skipwhite skipnl
 syn match elixirGuard               '\<defguard\>\(:\)\@!'        nextgroup=elixirFunctionDeclaration        skipwhite skipnl
 syn match elixirPrivateGuard        '\<defguardp\>\(:\)\@!'       nextgroup=elixirPrivateFunctionDeclaration skipwhite skipnl
 syn match elixirModuleDefine        '\<defmodule\>\(:\)\@!'       nextgroup=elixirModuleDeclaration          skipwhite skipnl
@@ -187,6 +188,7 @@ syn sync minlines=2000
 hi def link elixirBlockDefinition            Define
 hi def link elixirDefine                     Define
 hi def link elixirPrivateDefine              Define
+hi def link elixirNumericalDefine            Define
 hi def link elixirGuard                      Define
 hi def link elixirPrivateGuard               Define
 hi def link elixirModuleDefine               Define