Exploiting `segments` in a way that scales

Related to #451, the use of the segments parameter is a powerful way of grouping and multiplying your validation tests. Whilst having a custom label is useful to be able to see the segments at a glance, this could cause very large validation reports that are difficult to navigate and parse.

As a starting point, it would be useful to set a global segmentation scheme in create_agent() (much like actions), which will then apply to every validation function. Perhaps a way of overriding this for specific validation checks (e.g. setting to NULL) would be needed too.

When it comes to organising this in the report, would it be possible to split the HTML output into sections for each segment (or even better, tabs)? My use case is monthly reports spanning years, and I want to be able to see the issues within specific months.

I have a feeling I am just scratching the surface of what could be done here, and would be keen to hear others' thoughts.