Add @before hook

[bakerkretzmar]

This PR adds a new @before hook, identical to @after except that it runs before each task. Like @after and @error it receives the name of the task.

@before
    if ($task === 'finalize_release') {
        echo "Almost done!";
    }
@endbefore

My use case for this is outputting a friendly description of each task in a long story as it runs, without having the commands to echo out the messages cluttering up the beginning of every single task. I imagine there are lots of other uses for it too, like waiting for something to become available before starting a task, or making an initial measurement and then doing before/after comparisons when a task is finished.

If this PR is accepted I'll add this to the docs too.