`bundle config --local gemfile /foo/bar/MyGemfile` now works

- `bundle config` can be used to set a custom gemfile name for a
  project (specifying an absolute path)
- using other bundle commands will now search up parent directories
  for a .bundle dir and use the gemfile specified in .bundle/config if
  it is set
- does not require a file named 'Gemfile' to exist
- Revert "Correct bundle-config manpage"
  This reverts commit 0a81457.
- An added bonus: you can now both set and *delete* a global bundler
  setting from *anywhere*: `bundle config --delete foo`

### Manual Test:
In an empty directory `/foo/bar/`:
`echo "source 'https://rubygems.org'" > MyGemfile`
`echo "gem 'rake'" >> MyGemfile`
`bundle config --local gemfile /foo/bar/FooGemfile`
`bundle install`
`bundle show` # Should show the rake gem
`mkdir spec && cd spec && bundle show` # Should also show the rake gem