implement sub prefixes

[maxheld83]

from https://www.doi.org/doi_handbook/2_Numbering.html#2.2.1:

Prefix/suffix. Neither the Handle System nor DOI system policies, nor any web use currently imaginable, impose any constraints on the suffix, outside of encoding (see below). Handle syntax imposes two constraints on the prefix — both slash and dot are "reserved characters", with the slash separating the prefix from the suffix and the dot used to extend sub prefixes. The root administrator for the Handle System has reserved all prefixes starting with "10." (for example 10.1000, 10.1000.1, 10.23) for the IDF to use for DOI names.

Though I don't see any use case anywhere near, so won't be tackling this