config: add PELICAN_CONTENT

Allow the directory contents to be defined in the environment
diff --git a/pelicanconf.py b/pelicanconf.py
index 33152b0..d3b4035 100644
--- a/pelicanconf.py
+++ b/pelicanconf.py
@@ -26,7 +26,8 @@
 THEMEURL = SITEURL
 
 # Local path to the markdown source folder
-PATH = 'content'
+_PATH = os.getenv('PELICAN_CONTENT')
+PATH = _PATH
 
 # Local path to the current theme folder
 THEME = 'theme'