Add blog tagging

Squashed commit of the following:

commit 9bc1aa4cde0b4a5487630189727d30c378ba8aff
Author: Nadav Spiegelman <me@nadav.name>
Date:   Sat Jul 12 19:59:27 2014 -0400

    Implement tagging

commit 49ec31229a17f1ec0f1b42263a3fbaa934eb26d4
Author: Nadav Spiegelman <me@nadav.name>
Date:   Sat Jul 12 19:24:40 2014 -0400

    Add nav_link helper for adding active class

commit 5c5ac58a96695878788045eee13cad3111f597ac
Author: Nadav Spiegelman <me@nadav.name>
Date:   Sat Jul 12 18:32:40 2014 -0400

    Move nav list style to its own partial

commit 83f95e9c0ed03477ff4c270f06e6207f6ba78d3d
Author: Nadav Spiegelman <me@nadav.name>
Date:   Sat Jul 12 18:30:06 2014 -0400

    Consolidate article tags