add verticalLine() url function

hi. i've written a verticalLine() function for graphite's url api
which draws a vertical line on the graph at a specific
timestamp specified by the first argument function, 
in the same time formats as &from= and &until=, e.g.

&target=verticalLine("12:3420131108","event","blue")

this will be handy for quickly marking ad-hoc events 
on graphs without having to think of where it belongs
in the namespace and cluttering the database.

it seems like at least one other person wanted this functionality:
https://answers.launchpad.net/graphite/+question/187677