Add support for ctags in Elixir

Exuberant ctags has no native support for Elixir at this point. This
configuration is sourced from https://github.com/mmorearty/elixir-ctags.

ctags

@@ -1,2 +1,15 @@
 --regex-ruby=/(^|[:;])[ \t]*([A-Z][[:alnum:]_]+) *=/\2/c,class,constant/
 --regex-ruby=/^[ \t]*attr_(reader|writer|accessor) (:[a-z0-9_]+, )*:([a-z0-9_]+)/\3/A,attr/
+--langdef=Elixir
+--langmap=Elixir:.ex.exs
+--regex-Elixir=/^[ \t]*def(p?)[ \t]+([a-z_][a-zA-Z0-9_?!]*)/\2/f,functions,functions (def ...)/
+--regex-Elixir=/^[ \t]*defcallback[ \t]+([a-z_][a-zA-Z0-9_?!]*)/\1/c,callbacks,callbacks (defcallback ...)/
+--regex-Elixir=/^[ \t]*defdelegate[ \t]+([a-z_][a-zA-Z0-9_?!]*)/\1/d,delegates,delegates (defdelegate ...)/
+--regex-Elixir=/^[ \t]*defexception[ \t]+([A-Z][a-zA-Z0-9_]*\.)*([A-Z][a-zA-Z0-9_?!]*)/\2/e,exceptions,exceptions (defexception ...)/
+--regex-Elixir=/^[ \t]*defimpl[ \t]+([A-Z][a-zA-Z0-9_]*\.)*([A-Z][a-zA-Z0-9_?!]*)/\2/i,implementations,implementations (defimpl ...)/
+--regex-Elixir=/^[ \t]*defmacro(p?)[ \t]+([a-z_][a-zA-Z0-9_?!]*)\(/\2/a,macros,macros (defmacro ...)/
+--regex-Elixir=/^[ \t]*defmacro(p?)[ \t]+([a-zA-Z0-9_?!]+)?[ \t]+([^ \tA-Za-z0-9_]+)[ \t]*[a-zA-Z0-9_!?!]/\3/o,operators,operators (e.g. "defmacro a <<< b")/
+--regex-Elixir=/^[ \t]*defmodule[ \t]+([A-Z][a-zA-Z0-9_]*\.)*([A-Z][a-zA-Z0-9_?!]*)/\2/m,modules,modules (defmodule ...)/
+--regex-Elixir=/^[ \t]*defprotocol[ \t]+([A-Z][a-zA-Z0-9_]*\.)*([A-Z][a-zA-Z0-9_?!]*)/\2/p,protocols,protocols (defprotocol...)/
+--regex-Elixir=/^[ \t]*Record\.defrecord[ \t]+:([a-zA-Z0-9_]+)/\1/r,records,records (defrecord...)/
+--regex-Elixir=/^[ \t]*test[ \t]+\"([a-z_][a-zA-Z0-9_?! ]*)\"*/\1/t,tests,tests (test ...)/