add @spell to elixirString

This allows strings to be spell-checked by vim's `spell` feature

Author: jbodah

syntax/elixir.vim

@@ -81,10 +81,10 @@ syn region elixirStruct matchgroup=elixirStructDelimiter start="%\(\w\+{\)\@=" e
 
 syn region elixirMap matchgroup=elixirMapDelimiter start="%{" end="}" contains=ALLBUT,@elixirNotTop
 
-syn region elixirString matchgroup=elixirStringDelimiter start=+\z('\)+ end=+\z1+ skip=+\\\\\|\\\z1+ contains=@elixirStringContained
-syn region elixirString matchgroup=elixirStringDelimiter start=+\z("\)+ end=+\z1+ skip=+\\\\\|\\\z1+ contains=@elixirStringContained
-syn region elixirString matchgroup=elixirStringDelimiter start=+\z('''\)+ end=+^\s*\z1+ contains=@elixirStringContained
-syn region elixirString matchgroup=elixirStringDelimiter start=+\z("""\)+ end=+^\s*\z1+ contains=@elixirStringContained
+syn region elixirString matchgroup=elixirStringDelimiter start=+\z('\)+ end=+\z1+ skip=+\\\\\|\\\z1+ contains=@Spell,@elixirStringContained
+syn region elixirString matchgroup=elixirStringDelimiter start=+\z("\)+ end=+\z1+ skip=+\\\\\|\\\z1+ contains=@Spell,@elixirStringContained
+syn region elixirString matchgroup=elixirStringDelimiter start=+\z('''\)+ end=+^\s*\z1+ contains=@Spell,@elixirStringContained
+syn region elixirString matchgroup=elixirStringDelimiter start=+\z("""\)+ end=+^\s*\z1+ contains=@Spell,@elixirStringContained
 syn region elixirInterpolation matchgroup=elixirInterpolationDelimiter start="#{" end="}" contained contains=ALLBUT,elixirKernelFunction,elixirComment,@elixirNotTop
 
 syn match elixirAtomInterpolated ':\("\)\@=' contains=elixirString