Support for search replace

[ahall]

[gunnsth]

Example for search replace now available: https://github.com/unidoc/unidoc-examples/blob/v3/pdf/text/pdf_search_replace.go

[gunnsth]

The newest code for this: https://github.com/unidoc/unipdf-examples/blob/master/text/pdf_search_replace.go In the future it would be nice to have a way to do the search / replace easily with a high level interface. The extractor package could provide the search function and then can either modify the content stream or use creator package to do the replacement.

[gunnsth]

Related to #24 redaction ticket as redaction is basically removing the underlying text from the content and putting some rectangles over it and connection with Redaction annotations.