Add a SMIE rule function for "def" tokens.

This function now explicitly sets the indentation level of lines
beginning with "def" to 2. Formerly, this token returned `nil` (the
default, because there was no matching pattern in `elixir-smie-rules`),
which caused SMIE to call `smie-indent-after-keyword` to calculate the
indentation. This function was returning the incorrect value which was
causing the inappropriate alignment to the `.`.

Author: mattdeboard

elixir-smie.el

@@ -295,7 +295,8 @@ Return non-nil if any line breaks were skipped."
     (`(:after . "OP")
      (unless (smie-rule-sibling-p)
        elixir-smie-indent-basic))
-    (`(:before. "OP")
+    (`(:before . "def") elixir-smie-indent-basic)
+    (`(:before . "OP")
      ;; FIXME: Issue #5: This should prevent comments on lines before
      ;; continuation lines from causing indentation messed-upness, but
      ;; for some reason SMIE doesn't look this far when there's a