Add “Ignore warning” option to cabal check

* Introduce `-i/--ignore` command-line option.
    e.g.  cabal check --ignore=missing-upper-bounds  will not display
    “Missing upper bounds” warnings.

* Additionally, a filterPackageChecksById function is now exported
  in Distribution.PackageDescription.Check.Warning; this can be
  used by third party tools.

* Add CheckExplanationIDString to `cabal check` messages.
    e.g. from
      Error: The 'license' field is missing or is NONE.
    to
      Error: [license-none] The 'license' field is missing or is NONE.

    This makes using the cabal check `--ignore` option more immediate.

* Spool `MissingField` into separate constructors.
  Introducing five new constructors for `CheckExplanation`
    MissingFieldCategory
    MissingFieldMaintainer
    MissingFieldSynopsis
    MissingFieldDescription
    MissingFieldSynOrDesc
  This provides better ergonomic for `cabal check --ignore` and makes
  it easier to update the manual if the need arises.

* Add tests for desiderable `--ignore` string qualities (not too long,
  without too many '-', unique).

* Warn when `--ignore` string is not recognised.
    Also add a test for this.

* Add documentation.

* Add changelog.