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 the non-default gemfile
- works without a file named "Gemfile" in the current or parent(s)
  directory
- Revert "Correct bundle-config manpage"
  This reverts commit 0a814573363d48f8cd860d672f1c935fe1d89fca.