Add the possibility to turn off the automatic serial update.

Some users may want to explicitly update the DNS serial number. Instead
of forcing them to update on each write, a new configuration option is
used so they can chose turn it off.

README.md

@@ -17,3 +17,9 @@ I know this not the first vim plugin available to update a DNS-zone serial numbe
 * it lacks functionalities.
 
 It chose not to fork the original plugin but to write a new one from scratch mainly for legal purposes, but also because I did not found the code as simple as I expected.
+
+
+Configuration
+-------------
+
+You can turn off the automatic serial update by setting `let g:dnsserial_auto_update = 0` in your vimrc.

ftplugin/bindzone.vim

@@ -13,7 +13,13 @@
 " limitations under the License.
 
 
-augroup dnsserial
+if !exists('g:dnsserial_auto_update')
+    let g:dnsserial_auto_update = 1
+endif
+
+if g:dnsserial_auto_update
+    augroup dnsserial
         autocmd!
         autocmd BufWritePre <buffer> DNSSerialUpdate
-augroup END
+    augroup END
+endif