Added `brew info --path` command

Prints out one or more paths for the supplied formulae if they have been
installed on the user's system; useful when brew is used as part of a
shell script and the $PATH needs to be modified / environment variables
set.

From searching stack overflow, it would appear this feature has been requested once or twice; if the maintainers of this project thing there's value in adding this feature then I'm more than happy to follow up with test coverage.

Thanks! :)