Allow title attribute on sl-relative-time to override the default title attribute

[rikbosch]

We'd like to use the title attribute to display the full date and time when a user hovers the <sl-relative-time> component.

Currently it will just render the inner text as title of the inner <time> element.

[gengns]

Before 2.15.1 it was automatic:
<sl-relative-time date="2020-07-15T09:17:00-04:00"></sl-relative-time>
https://jsfiddle.net/mqun8jog/

Then, if you check the changelog while trying to fix sl-relative-time, it became what you said:
https://jsfiddle.net/1ezw5gy7/

After 2.18.0 we lost the original automatic approach:
https://jsfiddle.net/1ezw5gy7/1/

Now, we have to set a title with the date we want:
<sl-relative-time title="2020-07-15T09:17:00-04:00" date="2020-07-15T09:17:00-04:00"></sl-relative-time>