Dangling anchor links with German Umlauts when used with vuepress

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Reproduction

When using glossarify-md with vuepress a glossary term

## Der Tod und das Mädchen
Classic composition by Franz Schubert (The Death and the Maiden).

will be correctly found in text but the link in the text doesn't link to the glossary term.

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This is an issue with the Markdown-To-HTML translation of vuepress, or to be more precise with the slugify algorithm it uses. It will replace characters äöü with aou in HTML anchor names. So instead of

<a name="der-tod-und-das-mädchen">...</a>

it generates

<a name="der-tod-und-das-madchen">...</a>

This isn't really a bug with glossarify-md but affects any anchor names with unicode characters, no matter whether glossarify-md is used or not.

Solution: using the same slugifier with glossarify-md and vuepress

Vuepress allows to pass an external slugify function via option markdown.slugify. glossarify-md >=v1.1.2 will export a getSlugger() method. vuepress users can pass its return value to that option in their.vuepress/config.js.

This will affect how anchor names and URLs are constructed in general and will also modify vuepress' default behavior with regards to unicode characters in URLs or URL fragments (#fragment). Be aware of this if you have links in your documents where you previously "slugified" manually, e.g. by translating ä to a by hand when linking to a section header for example.

Also when removing glossarify-md any time later you may face some fragment refs no longer working. glossarify-md internally uses github-slugger. When removing glossarify-md you might want to use github-slugger directly to keep your links working. For more information on this issue and vuepress integration see https://github.com/about-code/glossarify-md/blob/master/doc/vuepress.md#configure-vuepress.

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vuejs/vuepress#1815