Add support for transliteration of German "umlaute" in slugs

[erikhofer]

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

[tech4him1]

We use a package -- https://github.com/andrewrk/node-diacritics -- to strip accents, so that is probably the best place to submit changes. If you have a better alternative or option, though, we can discuss here.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[CanRau]

One way might be to handle those before stripping diacritics

[luksak]

I'd love to see this solved. There is an issue for this against node-diacritics: andrewrk/node-diacritics#35

While researching, I realized that we are using a not well-maintained (latest commit is from 2017) and not that popular package. There are others that support German umlauts, have much more recent commits are a lot more popular:

• https://www.npmjs.com/package/slugify (by far most popular, not conform to RFC3986, no Chinese support, Typescript support)
• https://www.npmjs.com/package/speakingurl (conforms to RFC3986, has Typescript declarations)
• https://www.npmjs.com/package/limax (less popular, but depends on speakingurl and therfore is conforms to RFC3986, Typescript support)

I'd suggest switching to a package to has better transliteration support and is well maintained.