support more flexible assertions

[talentedmrjones]

Currently the expect API matches based on strict equality. I propose to replace expect: with assert: and provide functions such as !eq, !gt, !lt, !regex etc to each field as such:

tests:
- url: string
  assert:
    status: !eq 200 # or !gt 200
    body:
      text: !regex "foo" # to find any match, or !regex "^foo" to starts with match