Improve flyspell support

+ Make flyspell more selective about what to scan in markdown buffers.
+ Don't mark duplicates when proseline or langtool are present.
+ Add set-flyspell-predicate! for seeing mode-specific word predicates.

modules/lang/markdown/autoload.el

@@ -20,3 +20,23 @@
       (if (thing-at-point-looking-at +markdown--regex-del)
           (markdown-unwrap-thing-at-point nil 2 4)
         (markdown-wrap-or-insert delim delim 'word nil nil)))))
+
+;;;###autoload
+(defun +markdown-flyspell-word-p ()
+  "Return t if point is on a word that should be spell checked.
+
+Return nil if on a link url, markup, html, or references."
+  (let ((faces (doom-enlist (get-text-property (point) 'face))))
+    (or (and (memq 'font-lock-comment-face faces)
+             (memq 'markdown-code-face faces))
+        (not (cl-loop with unsafe-faces = '(markdown-reference-face
+                                            markdown-url-face
+                                            markdown-markup-face
+                                            markdown-comment-face
+                                            markdown-html-attr-name-face
+                                            markdown-html-attr-value-face
+                                            markdown-html-tag-name-face
+                                            markdown-code-face)
+                      for face in faces
+                      if (memq face unsafe-faces)
+                      return t)))))