Bump dependencies and implement "showDir" option

Bumped all the dependencies and dev dependencies.

Implemented a `-d` option corresponding to `node-ecstatic`'s "showDir" option.

Modified the `autoIndex` logic so that it compares with `'false'` instead of `false` (wasn't working otherwise). The new "showDir" option uses the same comparison.

All tests passing.

Closes #37, since the current version of `node-ecstatic` already supplies the desired behavior.