Jetpack Search: Add rel="nofollow" to widget filter URLs or provide filter for output

#### Is your feature request related to a problem? Please describe.
The Jetpack Search Widget has the option to display filters for your search. For example, you can add filters by Category so that after you do a search, you can apply a filter so that only certain categories will show up in the search results.

The problem we've seen with this is that Googlebot might see these as valid links and crawl them into eternity, resulting in some degraded performance.

Here is where the URL is output: https://github.com/Automattic/jetpack/blob/ba2dd6c30be137baad96aafb2466d0aed7e182e5/projects/plugins/jetpack/modules/search/class.jetpack-search-template-tags.php#L139

#### Describe the solution you'd like
Ideally, we could set these links to `rel="nofollow"` by default or at least add a filter to allow for users to change the output of the URLs to include a `nofollow` there.