[WIP] Add support for caret

[suyashmahar]

Attemps to add support for replace annotations:

The replace text annotation is made of a StrikeOut and a Caret annotation. The StrikeOut part is the line, and the little arrow is the Caret annotation. Since they appear as separate annotations, this hacky implementation modifies the content of the last StrikeOut annotation when it sees a Caret annotation.

However, I noticed that every annotation here appears twice:

https://github.com/0xabu/pdfannots/blob/f3d80dbcfbb15ffce5e3bc9ff45f269958925bfb/pdfannots/init.py#L396-L405

Attempts to resolve #61

• Caret annotations were introduced in the v1.5 spec. Page 616 of the PDF Reference 1.7
• Example PDF

@0xabu Do you have any thoughts on this?