Add the support for multiple formats.

Some people does not use the common YYYYMMDDXX format and/or does not
want to use the "; serial" comment right after the serial number. This
commit allow multiple formats to be defined, hense more users can use
the plugin. A huge consequence of this is that user can also define
additional custom formats depending on their needs.

plugin/dnsserial.vim

@@ -12,4 +12,28 @@
 " See the License for the specific language governing permissions and
 " limitations under the License.
 
+
+if !exists('g:dnsserial_custom_patterns')
+    let g:dnsserial_custom_patterns = []
+endif
+
+if !exists('g:dnsserial_patterns')
+    let g:dnsserial_patterns = [
+        \{
+            \'regex': '\(\d\{8}\)\(\d\+\)\s*;\s*\cserial',
+            \'matching': [
+                \{'type': 'date', 'fmt': '%Y%m%d'},
+                \{'type': 'integer', 'padding': 2, 'date_reset': 1}
+            \]
+        \},
+        \{
+            \'regex': '\(\d\+\)\s*;\s*\cserial',
+            \'matching': [
+                \{'type': 'integer'}
+            \]
+        \},
+    \]
+endif
+
+
 command! DNSSerialUpdate call dnsserial#DNSSerialUpdate()