This commit standardizes the various todos around the code a bit in a manner recognized by LDoc.
Besides drawing more attention by being displayed in the developer docs, they're also extractable with LDoc on the command line:
```sh
ldoc --tags todo,fixme *.lua
```
However, whether that particular usage offers any advantage over other search tools is questionable at best.
* and some random beautification
Split the html page download out of createEpub, so that createFromDescription
can pass in its own html for epub packaging (with optional image download).
Still pass in a url, so that relative urls can be made absolute.
Lift the Trapper:wrap call out of the individual article processing code,
so that articles are processed one by one, in order to:
* Avoid concurrent progress updates fighting over the UI dialog
* Avoid trying to download many images at the same time
Allow the user to specify whether to download images for each individual
feed specified in feed_config.lua. Default to false to stay closest to
existing behaviour.
