Improve loopback:relation for custom model name

Add an `other` option to model names list, if this option is
selected, prompt user to type a model name.
https://github.com/strongloop/generator-loopback/issues/35

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
Merge pull request #39 from clarkorz/allow-custom-relation-model

Improve loopback:relation for custom model name