Template.add `after=` parameter

[eseiver]

This pull request addresses #281 by introducing a new feature to the Template.add() method. Currently, when inserting a new parameter into a Template, the only option is to place it before an existing parameter. This PR enhances the functionality by adding support for inserting a parameter after an existing parameter, providing users with more flexibility.

Changes made:

• Added the 'after' parameter to the Template.add() method.
• Implemented the logic to insert the new parameter after the specified existing parameter.
• Created two appropriate test cases to validate the new feature, which are passing successfully.

Example:

>>> template = mwparserfromhell.parse("{{foo|bar|baz|eggs=spam}}")
>>> template.add("new_param", "value", after="baz")
>>> print(template)
{{foo|bar|baz|new_param=value|eggs=spam}}

[earwig]

Thanks a lot!