WIP: New interface proposal for numerical formatting: fmt and fmt_default!

[tbreloff]

I don't intend this to be merged as-is, but I want to start a discussion as to whether this approach will make sense. See the embedded TODO's for some ways that I would expect a final version to be different. You can see some example usage in tests/fmt.jl. Let me know your comments, and please... be brutal.

[ScottPJones]

Change the title to start with WIP:, that seems to be the convention, so people know that it is a work in progress.

[tonyhffong]

I'm philosophically ok with "yet another formatting api" as in cfmt. The symbol argument list is an acceptable alternative to the keyword arguments. Could you edit the README.md to advertise this properly?