charm: add VerifyWithCharms.

We don't yet verify the charm options. That will come later.
charm: adjust verifyRelation comment for review
Merge pull request #11 from rogpeppe/002-bundle-external-validation

charm: add VerifyWithCharms.
charm: change VerifyWithCharms to check charm options.

We also change it so that it takes a map[string]Charm
so it has access to the charm metadata. Strictly speaking,
that's more info than it needs (it doesn't use the charm
actions) but we think the API cleanliness makes it worthwhile.
charm: fix comments so they don't imply that verification order is important
Merge pull request #12 from rogpeppe/003-bundle-check-options

charm: change VerifyWithCharms to check charm options.