Support configuration of display mode

Users can set the default display mode in appsettings.json.

<style> </style>

ModeName	Description
Automatic	The continuous form of single page, facing or book view depending on the document's desired page layout
BookView	Dual page display (not continuous)
Continuous	Single page display in continuous mode
ContinuousBookView	Dual page display in continuous mode
ContinuousFacing	Dual page display in continuous mode
Facing	Dual page display (not continuous)
SinglePage	Single page display (not continuous)

from https://sumatrapdfcontrol.mcmonteiro.com/api/SumatraPDF.SumatraPDFControl.DisplayModeEnum.html