Boost Program Options accumulator type

After various requests, I've licensed my boost program_options extension code accumulator.hpp under the Boost Software License 1.0. It also now supports taking an argument on construction to store the value as well as the notifier function.

To recap: the accumulator type adds support for repeated options used to increment a particular value. Often this is used for verbosity, so that -v sets a verbosity level of one, -vvv a level of three.

The new accumulator.hpp is now stored on gitorious.

thanks!

Works nicely for me, including the new automatic -vvv style accumulation without callbacks.