Added text <watermark> with template

[Clouz]

In order to add page numbering function in the text watermark the below variable was added: {{.Page}} {{.Pages}} {{.Filename}}

[ajaxray]

Hello @Clouz ,

Many many thanks for your PR. ❤️ I had a small suggestion☝🏽, can you please check?

[Clouz]

@ajaxray thanks for your feedback, i will check in the next days.

[ajaxray]

Closing as this feature is already merged from #22