This commit adds a browsertrix profile option to the configuration. In
order to not require the passing of the browsertrix config to every
Archiver, the Archiver constructors (include the base) were modified to
accept a Storage and Config instance. Some of the constructors them pick
out the pieces they need from the Config, in addition to calling the
parent constructor. In order to avoid a circular import that this
created the Config object now defines the default hash function to use,
rather than having it be a static property of the Archiver class.
