added options object, with replace and separator

added ability to choose which characters are placed. for example, i'd like to let my slugs maintain domain/file names like `http://example.docs/repos/github.com`

figured this should convert the `repl` argument into an `options` object.

the replacement seemed to make sense as `options.replace`, especially since it's passed directly to a `.replace` method. and the original `repl` made more sense as a `options.separator`.

leme know what you think