feature(plugin): Document Viewer

[danielsogl]

Not ready yet

fix #1029

[GunaSekhar1]

Hi Daniel,

I am getting following error, when I try to open a PDF using document view plugin

plugins/cordova-plugin-document-viewer/www/sitewaertsdocumentviewer.js:344 Error in SitewaertsDocumentViewer.viewDocument():

"java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.io.File.exists()' on a null object reference
at de.sitewaerts.cordova.documentviewer.DocumentViewerPlugin.canGetFile(DocumentViewerPlugin.java:679)
at de.sitewaerts.cordova.documentviewer.DocumentViewerPlugin.doExecute(DocumentViewerPlugin.java:352)
at de.sitewaerts.cordova.documentviewer.DocumentViewerPlugin.access$000(DocumentViewerPlugin.java:49)
at de.sitewaerts.cordova.documentviewer.DocumentViewerPlugin$1.run(DocumentViewerPlugin.java:185)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
at java.lang.Thread.run(Thread.java:818)
"