Add defcustom for comment font-locking

The patch adds a `clojure-comment-regexp` defcustom that enable the user to
consider comments also the ones defined in a `(comment)` form.  The default
behavior is not changed, the comment reader macro is the only one considered a
comment.

Author: arichiardi

CHANGELOG.md

@@ -14,6 +14,7 @@
 * Make imenu recognize indented def-forms
 * New interactive command `clojure-cycle-when`.
 * New interactive command `clojure-cycle-not`.
+* New defcustom `clojure-comment-regexp` for font-locking `#_` or `#_` AND `(comment)` sexps.
 
 ## 5.6.1 (2016-12-21)
 

clojure-mode.el

@@ -614,15 +614,27 @@ If JUSTIFY is non-nil, justify as well as fill the paragraph."
   "Regexp matching the start of a comment sexp.
 The beginning of match-group 1 should be before the sexp to be
 marked as a comment.  The end of sexp is found with
-`clojure-forward-logical-sexp'.
+`clojure-forward-logical-sexp'.")
 
-By default, this only applies to code after the `#_' reader
-macro.  In order to also font-lock the `(comment ...)' macro as a
-comment, you can set the value to:
-    \"#_ *\\\\(?1:[^ ]\\\\)\\\\|\\\\(?1:(comment\\\\_>\\\\)\"")
+(defvar clojure--reader-and-comment-regexp
+  "#_ *\\(?1:[^ ]\\)\\|\\(?1:(comment\\_>\\)"
+  "Regexp matching both `#_' macro and a comment sexp." )
+
+(defcustom clojure-comment-regexp clojure--comment-macro-regexp
+  "Comment mode.
+
+The possible values for this variable are keywords indicating
+what is considered a comment (affecting font locking).
+
+    - Reader macro `#_' only - the default
+    - Reader macro `#_' and `(comment)'"
+  :type '(choice (const :tag "Reader macro `#_' and `(comment)'" clojure--reader-and-comment-regexp)
+                 (other :tag "Reader macro `#_' only" clojure--comment-macro-regexp))
+  :package-version '(clojure-mode . "5.7.0"))
 
 (defun clojure--search-comment-macro-internal (limit)
-  (when (search-forward-regexp clojure--comment-macro-regexp limit t)
+  "Search for a comment forward stopping at LIMIT."
+  (when (search-forward-regexp clojure-comment-regexp limit t)
     (let* ((md (match-data))
            (start (match-beginning 1))
            (state (syntax-ppss start)))