Add separator option and make it "_" by default

Should fix #30

I add a `separator` optionsinside an `hbs` option so each extractor could have its own options.
I think this line makes the `{separator: ' '}` above unnecessary? Also wondering about cases where people don't want any separator (so they'd use `''`). Doesn't seem likely, but it is possible. Maybe we should use `_.defaults`  here?