Feature: support GetWords()

[Banyc]

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We usually query a document word-by-word. However, docnet only supports character-oriented queries. Character-oriented queries are really cool since users can build word-oriented queries based on them. However, I believe it will be better if this common requirement could be implemented in docnet package.

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Thus, there is a need for GetWords function to return a list of words. Each word model has the location box and the text information, just like GetCharacters.

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[Modest-as]

The main reason why GetWords is not supported is because PDF documents have no concept of words. We expose all the info about the characters that one needs to do business logic specific clustering and so on. I am reluctant to add any sort of clustering to the core library itself because there will always be edge cases either due to document formatting or text direction and so on.

[talrand]

I also had a need to get words and lines of text as they appeared in the PDF.

I've created a small simple library to help with this https://github.com/talrand/DocnetExtended