Add in option to specify custom section separator, defaults to <br />

Hi,

we needed the ability to use a &nbsp; instead of <br /> when trying to show "28 minutes" (format: 'WDHMS',, significant: 1, compact: false) - without a custom layout which wouldn't take the significant-option into consideration.

The addition of this new option allows for more flexibility: the section separator can be changed on a per implementation basis.

Don't hesitate to get in touch if you have any remarks or comments - thanks in advance for merging the PR!

Kind regards,
David