Handle POST to /data/fs/[path]

Each line of the body is submitted to MongoDB as a "save" (which is effectively update/insert).

Given the MongoDB model, this actually supports creating a new collection, saving new documents to an existing collection, and updating existing documents (see the README).

All lines are parsed before any data is saved, so nothing is changed if the input is bad. However, partial writes can occur if the MongoDB driver gives an error.

Also added a command `save [path] [object]` to the REPL, and made the `ls` and `cd` commands actually work, for the most part.
I like this, although I think we should generalize the type of `values` so that not all data has to be in memory at once. i.e. you feed it a `Process[Task, RenderedJson]`?

Also, perhaps we name this `append` to reflect what it's doing now, and introduce a new method `save` which overwrites the contents of the file with the specified stream.