Allow bootstrapping non-default gemfile config

  - `bundle config --local gemfile FooGemfile` now works
     with or without the file existing
  - after config is set `bundle x` will use non-default gemfile
  - works without a "Gemfile" in the current directory
  - Revert "Correct bundle-config manpage"
    This reverts commit 0a814573363d48f8cd860d672f1c935fe1d89fca.