New feature: FETCH() permits http gets in dnsconfig.js (#1007)

add PANIC() and error-handled FETCH()

docs/_functions/global/FETCH.md

@@ -0,0 +1,44 @@
+---
+name: FETCH
+parameters:
+  - url
+  - args
+---
+
+`FETCH` is a wrapper for the [Fetch API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API). This allows dynamically setting DNS records based on an external data source, e.g. the API of your cloud provider.
+
+Compared to `fetch` from Fetch API, `FETCH` will call [PANIC](#PANIC) to terminate the execution of the script, and therefore DnsControl, if a network error occurs.
+
+Otherwise the syntax of `FETCH` is the same as `fetch`.
+
+`FETCH` is not enabled by default. Please read the warnings below.
+
+> WARNING:
+>
+> 1. Relying on external sources adds a point of failure. If the external source doesn't work, your script won't either. Please make sure you are aware of the consequences.
+> 2. Make sure DnsControl only uses verified configuration if you want to use `FETCH`. For example, an attacker can send Pull Requests to your config repo, and have your CI test malicious configurations and make arbitrary HTTP requests. Therefore, `FETCH` must be explicitly enabled with flag `--allow-fetch` on DnsControl invocation.
+
+{% include startExample.html %}
+{% highlight js %}
+var REG_NONE = NewRegistrar('none', 'NONE');
+var DNS_BIND = NewDnsProvider('bind', 'BIND');
+
+D('example.com', REG_NONE, DnsProvider(DNS_BIND), [
+  A('@', '1.2.3.4'),
+]);
+
+FETCH('https://example.com', {
+  // All three options below are optional
+  headers: {"X-Authentication": "barfoo"},
+  method: "POST",
+  body: "Hello World",
+}).then(function(r) {
+  return r.text();
+}).then(function(t) {
+  // Example of generating record based on response
+  D_EXTEND('example.com', [
+    TXT('@', t.slice(0, 100)),
+  ]);
+});
+{%endhighlight%}
+{% include endExample.html %}

docs/_functions/global/PANIC.md

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+---
+name: PANIC
+parameters:
+  - message
+---
+
+`PANIC` terminates the script and therefore DnsControl with an exit code of 1. This should be used if your script cannot gather enough information to generate records, for example when a HTTP request failed.
+
+{% include startExample.html %}
+{% highlight js %}
+PANIC("Something really bad has happened");
+{%endhighlight%}
+{% include endExample.html %}