Implemented Export as both PNG and PDF for Multipage Documents

[N-Shar-ma]

Please merge PR #957 before this as this branch is checked out from the #957 branch

Fixes #958

Changes made:

• for downloading, got all the outputPage DOM elements.
• looped through all of them and made a dataUrl from each.
• For PNG:
  • if there is a single page, exported 1 image with the user given file name
  • for multiple pages, exported an image for each page with the name being the user given file name followed by a hyphen and the page number. (example: user chose name doc for a 3 page document, so images generated will be doc-0.png, doc-1.png and doc-2.png)
• For PDF
  • created an array of all the dataUrls from all the pages' images
  • looped through this array, and added a new page in the pdf for each image
  • saved this single pdf by the user given file name

[smaranjitghose]

This makes the entire application much more usable! Nice work @N-Shar-ma .

w.r.t the UI, there is one suggestion I have in my mind, instead of having the new page appear below, it can occupy the location of the previous one and we can have the ability to toggle between pages with buttons on the side