Allow extra options to define their value type

[danwos]

Currently all extra_options for needs support string fields only. It would be good, if at least list types like for tags and links are supported.

Maybe other fields can be supported as well. E.g. numbers and dates

Drawbacks:

• Filtering gets more complex, as the user needs to know the exact option type for filter strings. amount == 2 and name == "Max Maier"
• The options must get the needed directives.type (e.g. unchanged) or a specific, internal type transformation must be performed. This may be needed, if the format (e.g. date) of a string must also be checked, which is not a feature of docutils.

Tech. concept

tbd