Fix {'keepopen':0} in NERDTreeCustomOpenArgs

[PhilRunninger]

Description of Changes

Closes #1168

When {'keepopen': 0} was specified in NERDTreeCustomOpenArgs the value of NERDTreeQuitOnOpen was overriding that value in the dictionary, not letting you define o to keep NERDTree open, and <CR> to close it.

This PR pushes the differentiating behavior of the NERDTreeQuitOnOpen setting up to an earlier point in the call stack, either keeping or resetting the `keepopen' value for the key mappings as appropriate.

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New Version Info

Author's Instructions

• Derive a new MAJOR.MINOR.PATCH version number. Increment the:
  • MAJOR version when you make incompatible API changes
  • MINOR version when you add functionality in a backwards-compatible manner
  • PATCH version when you make backwards-compatible bug fixes
• Update CHANGELOG.md, following the established pattern.

Collaborator's Instructions

• Review CHANGELOG.md, suggesting a different version number if necessary.
• After merging, tag the commit using these (Mac-compatible) bash commands:

  git checkout master
  git pull
  sed -n "$(grep -n -m2 '####' CHANGELOG.md | cut -f1 -d: | sed 'N;s/\n/,/')p" CHANGELOG.md | sed '$d'
  git tag -a $(read -p "Tag Name: " tag;echo $tag) -m"$(git show --quiet --pretty=%s)";git push origin --tags

[PhilRunninger]

This created the bug #1220.