Make the behavior of window splits consistent

[PhilRunninger]

Description of Changes

Closes #941
This PR addresses changes requested for #941, and expands on the work started there.

Closes #327
This PR forces NERDTree to respect the user's values for the splitright and splitbelow settings, and the g:NERDTreeWinPos variable. This changes the current behavior of the splits opened by NERDTree, so this will be a new MAJOR release - 6.0.0. The changes are as follows:

1. The window to be split will always be the previous window. This could be the one where :NERDTree (or :NERDTreeFocus or :NERDTreeToggle) was invoked. Or maybe a window command (such as 10<C-W>h) was used to jump directly to the NERDTree window. When in doubt, you can use the <C-W>p command to jump between the previous and current windows.
2. The g:NERDTreeWinPos variable will always be respected. This pertains to a NERDTree created when g:NERDTreeHijackNetrw is 1, and Vim is invoked with a directory, as in vim .. When splitting the window multiple times in this scenario, NERDTree will stay full-height on the desired side of the screen.
3. The vim setting splitright will be be respected. When performing a vertical split (s or gs), the previous window will be split to the right (splitright) or left (nosplitright) as directed by the user's setting.
4. The vim setting splitbelow will be be respected. When performing a horizontal split (i or gi), the previous window will be split above (nosplitbelow) or below (splitbelow) as directed by the user's setting.

These changes bring consistency to the NERDTree split commands, which previously had worked differently for vertical vs horizontal splitting.

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New Version Info

• Derive a new 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.
• Tag the merge commit, e.g. git tag -a 3.1.4 -m "v3.1.4" && git push origin --tags

[PhilRunninger]

@lifecrisis , would you have some time to look at this? It's not a huge change to the codebase, but it is a somewhat radical departure in terms of functionality. To test it, open vim both ways: both with a directory name argument, and without.

1. Once inside NERDTree, open several files, splitting both vertically and horizontally.
2. Go to a specific window you want to split, then open or focus NERDTree, and split-open a file.
3. In each case, see if the right window was split correctly.

There's not a big rush on this; I'll be exercising this branch over the coming days to see if anything shakes loose. I just would like another pair of eyes on it.