Support natural sorting on attributes

[JasperCraeghs]

Closes #354

The option :sort: of the item-attributes-matrix sorts items on attribute values alphabetically. This is good and desired as the default behavior.

A use case for natural sorting on attribute values came up. For example, sorting on values of the attribute effort like 11d and 5d.

This PR adds a new configuration value traceability_attributes_sort = {"effort": natsort.natsorted}. Any item-attributes-matrix that contains the effort attribute will sort on attribute values naturally.
This default value can be overridden in conf.py like all other configuration values.

[pietvr]

natsort introduced on effort

[JasperCraeghs]

There are other sorting methods besides alphabetical and natural. The configuration must be flexible enough to support those as well. https://natsort.readthedocs.io/en/master/api.html#natsort-api

traceability_attributes_natsort = {"effort"}
becomes
traceability_attributes_sort = {"effort": natsort.natsorted}

[JasperCraeghs]

Is the additional cyclomatic complexity acceptable? Reducing it by creating a separate function wouldn't be beneficial in my opinion. @Letme