Add support for Skribe document format.

* haunt/skribe.scm: New file.
* haunt/skribe/utils.scm: New file.
* haunt/reader/skribe.scm: New file.
* configure.ac: Check for guile-reader.
* Makefile.am (SOURCES): Add Skribe modules when guile-reader is
  available.
* example/haunt.scm: Include Skribe reader.
* example/posts/baz.skr: New file.
* README.md ("Requirements"): Mention guile-reader as optional dependency.

example/haunt.scm

@@ -1,16 +1,17 @@
-(use-modules (haunt site)
-             (haunt reader)
-             (haunt asset)
+(use-modules (haunt asset)
              (haunt builder blog)
              (haunt builder atom)
-             (haunt builder assets))
+             (haunt builder assets)
+             (haunt reader)
+             (haunt reader skribe)
+             (haunt site))
 
 (site #:title "Built with Guile"
       #:domain "example.com"
       #:default-metadata
       '((author . "Eva Luator")
         (email  . "eva@example.com"))
-      #:readers (list sxml-reader html-reader)
+      #:readers (list skribe-reader sxml-reader html-reader)
       #:builders (list (blog)
                        (atom-feed)
                        (atom-feeds-by-tag)

example/posts/baz.skr

@@ -0,0 +1,29 @@
+(post
+ :title "Hello, Skribe!"
+ :date (make-date* 2015 10 09 23 00)
+ :tags '("foo" "bar" "baz")
+
+ (h1 [Hello!])
+
+ (p [This is a Skribe document!])
+
+ (p [Skribe is a ,(em [really]) cool document authoring format that
+     provides all the power of Scheme whilst giving the user a
+     means to write literal text without stuffing it into a string
+     literal.  If this sort of thing suits you, be sure to check out
+     ,(anchor "Skribilo" "http://www.nongnu.org/skribilo/"), too.])
+
+ (p [Here's a simple list generated by Scheme code:])
+
+ (ul (map li '("foo" "bar" "baz")))
+
+ (p [And here's a code snippet of how I build Haunt using GNU Guix:])
+
+ (source-code
+  "guix environment -l package.scm
+./configure
+make")
+
+ (p [And finally, here's an image:])
+
+ (image "/images/guile-banner.small.png"))