extsprintf: extended POSIX-style sprintf

Stripped down version of s[n]printf(3c). We make a best effort to throw an
exception when given a format string we don't understand, rather than ignoring
it, so that we won't break existing programs if/when we go implement the rest
of this.

This implementation currently supports specifying

* field alignment ('-' flag),
* zero-pad ('0' flag)
* always show numeric sign ('+' flag),
* field width
* conversions for strings, decimal integers, and floats (numbers).
* argument size specifiers. These are all accepted but ignored, since
Javascript has no notion of the physical size of an argument.

Everything else is currently unsupported, most notably: precision, unsigned
numbers, non-decimal numbers, and characters.

Besides the usual POSIX conversions, this implementation supports:

* %j: pretty-print a JSON object (using node's "inspect")
* %r: pretty-print an Error object

Example

First, install it:

# npm install extsprintf

Now, use it:

var mod_extsprintf = require('extsprintf');
console.log(mod_extsprintf.sprintf('hello %25s', 'world'));

outputs:

hello world

Also supported

printf: same args as sprintf, but prints the result to stdout

fprintf: same args as sprintf, preceded by a Node stream. Prints the result
to the given stream.