New features: DU() and getConfiguredDomains() to assist modifying domains (#800)

* Added DU() and getConfiguredDomains()

* Added docs for both new functions

* Added a space between option and its value

* Renamed "DU" to "D_EXTEND", adjusted docs

* Fixed: Changed old DU() calls to D_EXTEND()

docs/_functions/global/D_EXTEND.md

@@ -0,0 +1,30 @@
+---
+name: D_EXTEND
+parameters:
+  - name
+  - modifiers...
+---
+
+`D_EXTEND` adds records (and metadata) to a domain. The domain must have previously been defined by `D()`. `D_EXTEND()` behaves the same as `D()` in all other ways: The first argument is the domain name. See the documentation of `D` for further details.
+
+Example:
+{% include startExample.html %}
+{% highlight js %}
+D('domain.tld', REG, DnsProvider(DNS),
+  A('@', "127.0.0.1")
+)
+D_EXTEND('domain.tld',
+  A('@', "127.0.0.2")
+)
+{%endhighlight%}
+
+This will end up in following modifications:
+```
+******************** Domain: domain.tld
+----- Getting nameservers from: registrar
+----- DNS Provider: registrar...3 corrections
+#1: CREATE A domain.tld 127.0.0.1 ttl=43200
+#2: CREATE A domain.tld 127.0.0.2 ttl=43200
+#3: REFRESH zone domain.tld
+```
+{% include endExample.html %}

docs/_functions/global/getConfiguredDomains.md

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+---
+name: getConfiguredDomains
+parameters:
+  - name
+  - modifiers...
+---
+
+`getConfiguredDomains` getConfiguredDomains is a helper function that returns the domain names
+configured at the time the function is called. Calling this function early or later in
+`dnsconfig.js` may return different results. Typical usage is to iterate over all
+domains at the end of your configuration file.
+
+Example for adding records to all configured domains:
+{% include startExample.html %}
+{% highlight js %}
+var domains = getConfiguredDomains();
+for(i = 0; i < domains.length; i++) {
+  D_EXTEND(domains[i],
+    TXT('_important', 'BLA') // I know, not really creative.
+  )
+}
+{%endhighlight%}
+
+This will end up in following modifications:
+```
+******************** Domain: domain1.tld
+----- Getting nameservers from: registrar
+----- DNS Provider: registrar...2 corrections
+#1: CREATE TXT _important.domain1.tld "BLA" ttl=43200
+#2: REFRESH zone domain1.tld
+
+******************** Domain: domain2.tld
+----- Getting nameservers from: registrar
+----- DNS Provider: registrar...2 corrections
+#1: CREATE TXT _important.domain2.tld "BLA" ttl=43200
+#2: REFRESH zone domain2.tld
+```
+{% include endExample.html %}
+
+Example for adding DMARC report records:
+{% include startExample.html %}
+This example might be more useful, specially for configuring the DMARC report records. According to DMARC RFC you need to specify `domain2.tld._report.dmarc.domain1.tld` to allow `domain2.tld` to send aggregate/forensic email reports to `domain1.tld`. This can be used to do this in an easy way, without using the wildcard from the RFC.
+
+{% highlight js %}
+var domains = getConfiguredDomains();
+for(i = 0; i < domains.length; i++) {
+	D_EXTEND("domain1.tld",
+		TXT(domains[i] + '._report._dmarc', 'v=DMARC1')
+	);
+}
+{%endhighlight%}
+
+This will end up in following modifications:
+```
+******************** Domain: domain2.tld
+----- Getting nameservers from: registrar
+----- DNS Provider: registrar...4 corrections
+#1: CREATE TXT domain1.tld._report._dmarc.domain2.tld "v=DMARC1" ttl=43200
+#2: CREATE TXT domain3.tld._report._dmarc.domain2.tld "v=DMARC1" ttl=43200
+#3: CREATE TXT domain4.tld._report._dmarc.domain2.tld "v=DMARC1" ttl=43200
+#4: REFRESH zone domain2.tld
+```
+{% include endExample.html %}