filter -> whitelist

Signed-off-by: Derek Richard <drichard@pivotallabs.com>
Allow commenting when the requester is whitelisted

Dredd will comment on the pull request to reaffirm the whitelisted status of
the requester with the provided template if a whitelisted_template
configuration option is provided.

Signed-off-by: Derek Richard <drichard@pivotallabs.com>