use slugify to generate anchors

[oupala]

metalsmith-permalinks switched to slugify for its last version.

Why not switch to slugify either so that slugs are consistent between url slugs and anchor slugs.

Example:

https://server/slug-by-metalsmith-permalinks/#slug-by-metalsmith-headings-identifier

It could be great!

Currently, é chars are nos converted to e by metalsmith-headings-identifier.