Updating how the assets path is calculated

Per #139, this updates how the assets path is calculated

When `options.assets` ends with a trailing slash,
the `assets` path will have a trailing slash

When `options.assets` does not end with a trailing slash,
the `assets` path will not have trailing slash

When the `assets` path resolves to the same path
as the current page destination and `options.assets`
ends with a trailing slash, then `assets` will be `./`
otherwise `assets` will be blank `''`