registerAll method
Adds and (if a host was provided) installs every plugin in plugins,
preserving order.
Implementation
Resolvable<Unit> registerAll(Iterable<TPlugin> plugins) {
final seq = TaskSequencer();
for (final plugin in plugins) {
seq.then((_) => register(plugin).then((e) => Some(e)));
}
return seq.completion.toUnit();
}