Passages tags added into data-tags html attribute

Implements the idea from: http://www.glorioustrainwrecks.com/node/5013
suggested by Leon Arnott

Enables passage/tag specific CSS2 selectors: e.g.
[data-tags~=sometag] { color:#f00; }

Change-Id: I138c64ee90fb745727b1482bc4fba50eb4bb7882
Signed-off-by: Stormrose <eturnerx@gmail.com>
data-tags attribute only created if passage tags exist

Change-Id: Ic2a76f49ed246583912612ab01f9a2544f8307b5
Signed-off-by: Stormrose <eturnerx@gmail.com>
Re-enabled tooltips at request of Philip Sutton

Change-Id: Id5816670e18e3c54788e08a12694476d1ee9948c
Signed-off-by: Stormrose <eturnerx@gmail.com>