window_cmd option to allow user to control the window position and behaviour

[LintaoAmons]

{
...
  "window_cmd": "edit", // 'vsplit' | 'split' | 'edit' | 'tabedit' | 'rightbelow vsplit'. Can use rightbelow or topleft etc. as modifier
...
}

Thanks @kohane27 for the pr

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This discussion was created from the release window_cmd option to allow user to control the window position and behaviour.