Watermark formatting/styling

[liborm85]

This add ability to watermark formatting and fix issues #466, #476, #366 and partly #187 (without images).

Example without any own style:

watermark: 'test watermark'

Example with style:

watermark: {text: 'test watermark', color: 'blue', opacity: 0.3, font: 'Courier', bold: true, italics: true}

Default values:

• color: black
• opacity: 0.6
• font: used from defaultStyle or if not defined is used Roboto
• bold: false
• italics: false

[stephg]

Hey guys, what is missing on this PR? Watermarks are just broken in current master...

[darioxlz]

how i can put a image style watermark in pdf export?

[ADesai-1]

how i can put a image style watermark in pdf export?

I have the same issue. I want to put a image as a watermark