Support conversion from and to Textract JSON

[scottschreckengaust]

Textract has an output results format in JSON.

https://docs.aws.amazon.com/textract/latest/dg/textract-dg.pdf

Specifically, the three types of analysis, https://docs.aws.amazon.com/textract/latest/dg/how-it-works-analyzing.html for the categories:

1. text,
2. forms, and
3. tables

[stweil]

Conversion from Textract to PAGE XML was now added with pull request #160.

[bertsky]

Alas, the new converter is still incomplete, so

• forms, and
• tables

do not work yet. See slub/textract2page#2

[bertsky]

Update: tables work now, but the converter submodule needs to be updated here

[kba]

Update: tables work now, but the converter submodule needs to be updated here

I've updated the vendor submodules, including textract2page in #166. The tables branch is not yet merged to master though and I think there are files missing to properly run the tests.