Allow reporters to iterate over the collection of tasks?

Looking at the way reporters are set up, they are a little limited; the module itself handles all of the iteration through each group, which can be slightly confusing when you try to output something more than just a header before each group. The markdown reporter is slightly flawed in this regard when it comes to nesting the headers properly.

If each reporter were to be passed the `tasks` variable itself, which has all of the tasks grouped, filtered etc, then it can choose whether it wants to run some code before/after the whole series of tasks, or each task individually. It allows for more flexibility, with the drawback of having to write more code for if/when you want a custom reporter.

Slightly related; the default reporter may be kept around, but a nicer reporter should probably be made that is more informative. It shouldn't have to be necessary for someone to read this project's README in order to understand what `>`, `->` and `=>` mean.