Abstract out the nsupdate provider logic and implement a new resource_record
type around it which applies no semantic value to its title.
Also: support SPF and TXT records, include isrequired on required parameters
(though it doesn't seem to work as advertised)
Add the tsig_param when invoking dig using the -y command line option. This
allows dig and nsupdate to operate in the same view when views are matched by
tsig signature.
