Add hOCR output format

[sliedes]

This change adds rudimentary hOCR output support. Notes:

• Currently it just adds bounding boxes, not baselines (which are also supported) to the hOCR output

• It doesn't add any semantic layout stuff; instead, it just represents each word as an ocrx_word

• Some of the metadata could be improved, such as adding the real image name and perhaps EasyOCR version number

• I didn't check if EasyOCR supports multipage inputs; this will certainly break with those if it does

• I left this comment in the source code; I'm not sure what to do with it (probably shouldn't be enabled by default):

# In order to get a browser-renderable HTML file, you can add this before the closing </body> tag:
#
# <script src="https://unpkg.com/hocrjs"></script>

Other than that, I validated the output with hocr-check from https://github.com/ocropus/hocr-tools and also checked that it validates as XHTML.