Added a plugin system, based on hint.

+ In Text.Pandoc.Definition, added processIn, processInM,
  and queryIn, and deprecated processPandoc and queryPandoc
  for these more general functions, which are useful in writing
  plugins.

+ Added module Text.Pandoc.Plugins.

+ Added a --plugins option to Main, and code to run the parsed pandoc
  document through all the plugins.

+ Provided five sample plugin files in the plugins/ directory.

+ Documented --plugin in the pandoc man page and README.


git-svn-id: https://pandoc.googlecode.com/svn/trunk@1519 788f1e2b-df1e-0410-8736-df70ead52e1b

plugins/IncludeFilePlugin.hs

@@ -0,0 +1,19 @@
+module IncludeFilePlugin (transform) where
+import Text.Pandoc
+import Text.Pandoc.Shared
+import Control.Monad
+
+-- This plugin allows you to include the contents of an
+-- external file in a delimited code block like this:
+--
+-- ~~~ {include="filename"}
+-- ~~~
+--
+-- Trailing newlines are trimmed.
+
+transform :: Block -> IO Block
+transform cb@(CodeBlock (id, classes, namevals) contents) =
+  case lookup "include" namevals of
+       Just f     -> return . (CodeBlock (id, classes, namevals) . stripTrailingNewlines) =<< readFile f
+       Nothing    -> return cb
+transform x = return x