Allow for specification of files to be included in server pack. Extends "Folders to be included in server pack".

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• What is the current behavior?
  Only folders inside the modpack can be specified for inclusion in the server pack. The user can not specify files to be included in the to-be-created server pack.

• What is the wanted/requested behavior?
  Allow users to specify files inside their modpack to be included in the to-be-created server pack.

• What are your reasons for making this feature request? Provide solid arguments on why I should start working on it.
  User jackboi03 on Discord messaged me with the request of adding a feature to include a whitelist in the server pack.
  Server packs created should be uploadable to Modpack sharing sites like CurseForge and anyone who downloads these server packs should be able to use them right from the get go.
  Including a pre-defined whitelist goes against that, as it would automatically restrict who can join the server created from the server pack in question.
  However. That being said. I may be able to expand a feature so it would allow users to include a whitelist, should they wish so.
  I'll see whether I can make it so a combination of "path/to/sourcefile:/path/to/server/pack/file" would allow the user to include any specific file as well.

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Haven't gotten any negative feedback so far and all local tests worked flawlessly. Will probably merge and release in the next couple of days.

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Available as of Release 2.1.0