initial ipv6 support

this adds and waits for AAAA records based on setting. we have to wait
for both A and AAAA because we don't know if the user is accessing via
IPv4 or IPv6. For Let's Encrypt, IPv6 is preferred (but not sure if it
retries if IPv6 is unreachable).

part of #264

src/dns/namecheap.js

@@ -37,7 +37,7 @@ function injectPrivateFields(newConfig, currentConfig) {
 async function getQuery(domainConfig) {
     assert.strictEqual(typeof domainConfig, 'object');
 
-    const ip = await sysinfo.getServerIPv4();
+    const ip = await sysinfo.getServerIPv4(); // only supports ipv4
 
     return {
         ApiUser: domainConfig.username,