Refresh font-lock rather than recomputing from defaults

We can use font-lock-flush now to simply refresh the fontification of
the current part of the buffer.  Previously, we needed to use the more
heavy-handed font-lock-refresh-defaults, because we were regenerating
the list of font-lock-keywords from scratch and reloading each time.
Now, keywords are added and removed as needed, or else are based on
functions with feature-specific conditionals.

We also don’t need to check that font-lock-mode is enabled, because
font-lock-flush does that already.

The result is that user-defined keywords such as these are not
clobbered:

(font-lock-add-keywords nil '(("\\W@\\w+" . font-lock-variable-name-face)))

Closes GH-222.

markdown-mode.el

@@ -8701,8 +8701,7 @@ or span."
   (interactive)
   (when (member major-mode '(markdown-mode gfm-mode))
     ;; Refontify buffer
-    (when (and font-lock-mode (fboundp 'font-lock-refresh-defaults))
-      (font-lock-refresh-defaults))
+    (font-lock-flush)
     ;; Add or remove hooks related to extensions
     (markdown-setup-wiki-link-hooks)))