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.

### Manual Test:
In an empty directory:
`echo "source 'https://rubygems.org'" > FooGemfile`
`echo "gem 'rake'" >> FooGemfile`
`bundle config --local gemfile FooGemfile`
`bundle install`
`bundle show`
Should show the rake gem