Add text watermark with template

[AnkurGel]

Builds on top of https://github.com/ajaxray/markpdf/pull/14 with feedback from @ajaxray in mind

Approach:

1. Check if given watermark is a template with isWatermarkATemplate
2. If a template, dynamically change the text in Paragraph object by after executing template in iteration.
3. else, rely on static-text and continue with old behavior without doing extra work or creating extra objects.