Log level options

Most useful would be `--verbose`, `--quiet` might be nice too. Tasks would need a way to hook into this, some ideas would be:
1. expose a `Logger`
2. add a `Level level` param to `log`
3. Explicit `void verbose(String msg)` method

Depending on the solution for #78 it may go on `GrinderContext` or top-level.

Whichever option, it probably makes sense to use a `Logger` internally at least.

I'm planning to make it easy to set a log `Level` from command-line flags with unscripted: https://github.com/seaneagan/unscripted/issues/4