NEW FEATURE: Moving provider TYPE from dnsconfig.js to creds.json (#1500)

Fixes https://github.com/StackExchange/dnscontrol/issues/1457

* New-style creds.json implememented backwards compatible

* Update tests

* Update docs

* Assume new-style TYPE

pkg/js/parse_tests/041-newstyleproviders.js

@@ -0,0 +1,31 @@
+// Test old-style and new-style New*() functions.
+
+var REG1 = NewRegistrar("foo1");
+var CF1 = NewDnsProvider("dns1");
+
+var REG2a = NewRegistrar("foo2a", "NONE");
+var CF2a = NewDnsProvider("dns2a", "CLOUDFLAREAPI");
+
+var REG2b = NewRegistrar("foo2b", {
+    regmetakey: "reg2b"
+});
+var CF2b = NewDnsProvider("dns2b", {
+    dnsmetakey: "dns2b"
+});
+
+var REG3 = NewRegistrar("foo3", "MANUAL", {
+    regmetakey: "reg3"
+});
+var CF3 = NewDnsProvider("dns3", "CLOUDFLAREAPI", {
+    dnsmetakey: "dns3"
+});
+
+var REG1h = NewRegistrar("foo1h", "-");
+var CF1h = NewDnsProvider("dns1h", "-");
+
+var REG2bh = NewRegistrar("foo2bh", "-", {
+    regmetakey: "reg2bh"
+});
+var CF2bh = NewDnsProvider("dns2bh", "-", {
+    dnsmetakey: "dns2bh"
+});