Add command line option all

Use --all or -a when calling site-generator instead of special page
ALL to generate all the configured pages for a website.

README.md

@@ -26,7 +26,7 @@ Generate two specific pages:
 `site-generator index about`		
 
 Generate the whole site:
-`site-generator ALL`	
+`site-generator --all`	
 
 Generate pages based on the same template:
 `site-generator correspondent=1,3,5..10`