HTTP-probe should allow regexp testing of body

[Steve Kemp]

I'm currently testing the blogspam stats via:

  https://blogspam.net/xml/stats must run http with content '<spam>'

That works, but it would be better to test that actual numbers were returned.

To resolve this I'd suggest we allow:

  with regexp '<spam>^[0-9]+$</spam>'

The regexp option would work like the existing content option, but would be a regular expression which must match.