Allow multiple for other plural options (e.g. --import-mappings)

[ePaul]

PR checklist

• Read the contribution guidelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change.
  → bin/run-all-petstore, with no changes (apart from those who already happen in master, see Change all sample-generators for typescript-angular2-* to use YAML #5859 + Updating samples for Jersey2. #5861).
• Filed the PR against the correct branch: master for non-breaking changes.

Description of the PR

As discussed in #5147, all the options which previously had comma separated lists of values now also allow multiple values (or multiple key-value-pairs), as was previously (in #5191) already done for -D.
This affects those with key-value-pairs:

--additional-properties
--import-mappings
--instantiation-types
--reserved-word-mappings
--type-mappings

and this option with a simple list of values:

--language-specific-primitives

I also regenerated the output of help generate in README.md, which was already out-of-date before.