feat: add - as a substitution for . to plugin heuristics (+ improve docs)

[mrcjkb]

• If a plugin is called foo.bar, it would have to be put in a lua/foo/bar.lua file. This PR adds a heuristic, so it can be put in a lua/foo-bar.lua file.
• The documentation on what constitutes a valid config file name is not clear (see #13) so I have added a note on that.

[github-actions[bot]]

Review Checklist

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• For example, fix(installer): some installer bugfix

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If applicable:

• [ ] Tested
  • [ ] Tests have been added.
  • [ ] Tested manually (steps in PR description).
• [x] Updated documentation.