Rename 'spell_language' to 'spelling_language'

Discussion on the editorconfig standard seems to converge
on calling the property 'spelling_language' rather than
'spell_language' (see [1]). This commit adjusts both the
value that is expected to be found in .editorconfig files
as well as the internal variable that stores this property.

Note to editorconfig users: This commit changes the plugin's
behaviour in a non backwards compatible way. All editorconfig
files that contain the 'spell_language' property need to be
updated.

Note to spellchecker plugin developers: Since this commit
also updates the internal variable name, plugins need to
update this name as well.

[1] editorconfig/specification#41

edconf.lua

@@ -116,8 +116,8 @@ OPTIONS = {
     vis_set("tabwidth", value)
   end,
 
-  spell_language = function (value, file)
-    file.spell_language = value
+  spelling_language = function (value, file)
+    file.spelling_language = value
   end,
 
   insert_final_newline = function (value)