Fix tags generation when wiki extension has multiple dots

[MartyLake]

When using extension containing multiple dots, for example .wiki.txt, generated tags would contains a faulty links that ends with piece of the extension, for example .wiki, then vimwiki would try to go to a file that ends with .wiki.wiki.txt instead.

This patch solves the issue by removing the exact suffix of the extension instead of using the root filename modifier.

Steps for submitting a pull request:

• ALL pull requests should be made against the dev branch!
• Take a look at CONTRIBUTING.MD
• Reference any related issues.
• Provide a description of the proposed changes.
• PRs must pass Vint tests and add new Vader tests as applicable.
• Make sure to update the documentation in doc/vimwiki.txt if applicable,
  including the Changelog and Contributors sections.