Add possibility to provide custom providers

[Karl-Philipp Richter]

The built-in provider `hnorg`, `dyndns` and `noip` use specified URL and the `dyndns` are not always sufficient to configure the DDNS update queries (e.g. via HTTP(S)). It'd make sense to allow specification of providers as modules in files which can be referenced and used in `dyndns`.

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: dyndns 2014.0708-1
Uname: Linux 4.0.0-040000-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.14.7-0ubuntu8.4
Architecture: amd64
Date: Tue Apr 21 16:53:56 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-04-20 (0 days ago)
InstallationMedia: Ubuntu-Server 14.10 "Utopic Unicorn" - Release amd64 (20141022.2)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: dyndns
UpgradeStatus: No upgrade log present (probably fresh install)