Introduce custom bindings form for posts.

Use `bindings: name1 value1 ...` to bind special variables while
compiling a post.  Only a single bindings directive is supported in the
post metadata.

src/content.lisp

@@ -48,17 +48,32 @@
 ;; Content Types
 
 (defclass content ()
-  ((url  :initarg :url  :reader page-url)
-   (date :initarg :date :reader content-date)
-   (file :initarg :file :reader content-file)
-   (tags :initarg :tags :reader content-tags)
-   (text :initarg :text :reader content-text))
-  (:default-initargs :tags nil :date nil))
+  ((url      :initarg :url      :reader page-url)
+   (date     :initarg :date     :reader content-date)
+   (file     :initarg :file     :reader content-file)
+   (tags     :initarg :tags     :reader content-tags)
+   (text     :initarg :text     :reader content-text)
+   (bindings :initarg :bindings :reader content-bindings))
+  (:default-initargs :tags nil :date nil :bindings nil :code nil))
+
+(defun read-bindings-from-string (string)
+  "Read multiple pairs of values from STRING and return them as two LISTs
+of variables and values ready to be passed into PROGV."
+  (with-input-from-string (stream string)
+    (loop
+      for variable = (read stream nil)
+      for value = (read stream nil)
+      while variable
+      collect variable into variables
+      collect value into values
+      finally (return (cons variables values)))))
 
 (defmethod initialize-instance :after ((object content) &key)
-  (with-slots (tags) object
+  (with-slots (tags bindings) object
     (when (stringp tags)
-      (setf tags (mapcar #'make-tag (cl-ppcre:split "," tags))))))
+      (setf tags (mapcar #'make-tag (cl-ppcre:split "," tags))))
+    (when bindings
+      (setf bindings (read-bindings-from-string bindings)))))
 
 (defun parse-initarg (line)
   "Given a metadata header, LINE, parse an initarg name/value pair from it."

src/posts.lisp

@@ -7,11 +7,12 @@
   (:default-initargs :author nil))
 
 (defmethod initialize-instance :after ((object post) &key)
-  (with-slots (url title author format text) object
-    (setf url (compute-url object (slugify title))
-          format (make-keyword (string-upcase format))
-          text (render-text text format)
-          author (or author (author *config*)))))
+  (with-slots (url title author format text bindings) object
+    (progv (car bindings) (cdr bindings)
+      (setf url (compute-url object (slugify title))
+            format (make-keyword (string-upcase format))
+            text (render-text text format)
+            author (or author (author *config*))))))
 
 (defmethod render ((object post) &key prev next)
   (funcall (theme-fn 'post) (list :config *config*