Add a `MakeWriter` param to `HierarchicalLayer`

[davidbarsky]

Few changes:

• Added a MakeWriter parameter to HierarchicalLayer, which allows for customizing the destination out the printed output. Note that ansi-handling isn't handled automatically with MakeWriter, as a file destination might require the caller to disable ansi printing themselves.
• Re-organized lib.rs; there's now a dedicated format module that handles most of the pretty-printing.
• Added a stderr example (borrowed from https://github.com/tokio-rs/tracing/blob/master/examples/examples/attrs-args.rs), which demonstrates how to customize the output destination.
• I cut down on the required features of tracing-subscriber.

Resolves #9

cc: @flodiebold

[davidbarsky]

Here's a screenshot of the (slightly modified) stderr example, printing out a fibonacci sequence: