Allow user to set paper size for PDF command

[Michael-Bell]

Currently defaults to A4. Would be nice to have an optional argument for this.
Wkhtmltopdf has an argument for it:

  -s, --page-size <Size>              Set paper size to: A4, Letter, etc. (default A4)

Going through the source code leads me to believe the following sizes are supported:

• A0
• A1
• A2
• A3
• A4
• A5
• A6
• A7
• A8
• A9
• B0
• B1
• B2
• B3
• B4
• B5
• B6
• B7
• B8
• B9
• B10
• C5E
• Comm10E
• DLE
• Executive
• Folio
• Ledger
• Legal
• Letter
• Tabloid

[craig-davis]

That would be a fantastic option to have. I'm pretty busy lately and won't be able to add it. But I'd love to have a pull request for it, or will add you as a contributor!

[danielklim]

Addressed in #84. See #78 for more info.