Is your feature request related to a problem? Please describe.
If the title contains a German umlaut (äöü) and use clean_accents: true, they are converted to plain letters aou. While this is suitable for actual accents (e.g. é), it looks really odd for umlaute.

Describe the solution you'd like
In German there is an official rule for transliterating special characters:

ä -> ae
ö -> oe
ü -> ue
ß -> ss

The last rule is already covered by clean_accents: true. It would be nice to change the conversion of the remaining characters or provide an additional setting to enable it.

Describe alternatives you've considered
Manually editing the file name is possible but inconvenient. The ability to edit slugs as discussed in #445 would make this less of an issue.

Additional context
N/A