+ 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
