Revert "Fix support for dotfiles in data-sources"

#492 adds support for dotfiles in data sources, but unfortunately introduces a backwards incompatibility issue: sites likely currently rely on dotfiles being ignored, and this change would break their sites. A common dotfile is `.DS_Store`, which few sites (if any at all!) take into account.

This PR reverts the change. Having support for dotfiles is good, but in order to keep backwards compatibility, it should be optional, and opt-in.

Also see [this and later comments on #496](https://github.com/nanoc/nanoc/pull/496#issuecomment-63274338).

CC @andydrop