Add option to not re-evaluate specific script tags

Adding data-turbolinks-eval=false to a script tag will not re-evaluate
the JavaScript again on turbolinks page visits. This can be used to
avoid moving script tags to the <head> to improve uncached page load
times.
Merge pull request #214 from mariovisic/add_turbolinks_eval_flag

Add option to not re-evaluate specific script tags