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.

README.md

@@ -28,7 +28,7 @@ By default, each time you save a `bindzone` file, the script will look for the D
 Patterns
 --------
 
-In order to be detected, the DNS serial number must match the following pattern:
+In order to be detected, the DNS serial number must match one the following pattern:
 
 * `YYYYMMDDXX ; serial`
   - `YYYY` is the year (4 digits);
@@ -38,8 +38,65 @@ In order to be detected, the DNS serial number must match the following pattern:
   - the word `serial` is not case-sensitive;
   - there can be any number of blanks on each sides of the semicolon.
 
+* `XX ; serial`
+  - `XX` is any non-negative number (1 or more digits);
+  - the word `serial` is not case-sensitive;
+  - there can be any number of blanks on each sides of the semicolon.
+
 
 Configuration
 -------------
 
-You can turn off the automatic serial update by setting `let g:dnsserial_auto_update = 0` in your vimrc.
+You can set several configuration variables in your vimrc:
+
+* `g:dnsserial_auto_update`: Defines whether or not the serial is updated when the zone file is saved (default is 1, set it to 0 to disable).
+* `g:dnsserial_custom_patterns`: List of customs patterns that will be added to the default ones. Order matters, the first matching patters will be used. Customs patterns will be tested before the default ones.
+* `g:dnsserial_patterns`: List of default patterns. It is not advised to change it.
+
+
+Custom patterns
+---------------
+
+A pattern is defined by a dictionary with two keys: `regex` and `matching`.
+
+**regex**
+
+Contains the regular expression that will be used to search the document for the serial number. All the components of the serial number must be captured with parenthesis.
+
+**matching**
+
+This is a list of every components of the serial number. Each component is defined by a dictionary. The `type` key must be present and contain one of the allowed types. Depending on the type, several additional keys might be defined. Authorized types and their options are:
+
+* `raw`: Raw string.
+* `integer`: An integer that will be incremented.
+- `offset` (int): set the offset by which the integer is incremented. Default is 1.
+- `padding` (int): Force the integer to be 0-padded on the associated number of digits.
+- `date_reset` (bool): If set to 1 and a the serial contains a `date`, the integer will be reseted to 0 if the date is updated. Default is 0.
+* `date`: A formated date that will be updated to the current one.
+- `fmt` (string, mandatory) : the date format according to the `strftime()` specifications. See `:help strftime` for more details.
+
+**examples**
+
+A simple pattern matching a serial defined as an integer and followed by a comment starting by the word `serial` is:
+
+```json
+{
+  'regex': '\(\d\+\)\s*;\s*\cserial',
+  'matching': [
+    {'type': 'integer'}
+  ]
+}
+```
+
+The same example, but having the serial number starting by the current date (YYYYMMDD) and the integer padded on two digits:
+
+
+```json
+{
+  'regex': '\(\d\{8}\)\(\d\+\)\s*;\s*\cserial',
+  'matching': [
+    {'type': 'date', 'fmt': '%Y%m%d'},
+    {'type': 'integer', 'padding': 2, 'date_reset': 1}
+  ]
+}
+```