Implementation of the blog generator

This commit adds implementation of the blog generator, which
incorporates feed, index page and tags pages generators.
As for sense of beauty. 

1. Why in L112 you use list comprehensions instead of generator (like in L114) ?
2. Why you add useless empty lines: L113, L115. I believe the actions between L112-L117 are in the same group and shouldn't be separated by empty lines.
3. You did the right thing, that this action is placed last. It was a deliberate act or not? If not I suggest you to think about  it.
4. Generally we have a performance issue here. How to fix that?