`coffee DIR` executes `DIR/index.coffee`

Resolves #2496 and brings the CLI behavior for directories in line with redux, node and coco/LS. Runs either `index.coffee`, `index.litcoffee` or `index.coffee.md` (just like `require DIR` does already).

`coffee --watch DIR` watches and (re-)runs only the index file (and no longer all contained coffee files and subdirectories). This behavior is copied from coco/LS and provides a nice symmetry between invocations with and without `--watch`, but I'm not entirely sure about potential use cases...