Add support for stdin

[supersym]

I didn't do a pull request. You have a few waiting anyway.

I suggest something like, to stay in line with the rest of the code and conventions, to modify cli.coffee and append a switch e.g. -i for processing stdin.

 ['-i', '--stdin', 'accept input over standard input interface stdin']

Then add

codeIn = ""
stdin = process.openStdin()

# and under run...

if options.stdin
    # Buffer streams of data and concat to string until finished
    stdin.on 'data', (buffer) -> codeIn += buffer.toString() if buffer
    stdin.on 'end', -> log coffeecup.render codeIn

This will allow for something like

$ coffee -p -b myfile.litcoffee | js2coffee | coffeecup -i

As a little round-trip since coffeescript cli tool doesn't really allow a unmangle of the .litcoffee files yet.