Derive list of tsfile: link targets

[novoid]

Add an additional command line parameter similar to --tsfile-links $FILE. This $FILE gets populated with CSV content like:

2020-04-12-my-nice_image.jpeg; id:2020-04-12-article-id
2018-09-10T13.55.03 flower at a tree -- publicvoit cliparts.jpg; id:another-id

The entries refer to images which got linked using the tsfile-method. The CSV file might be handy for locating externally linked files for some reason.

Note: Determining broken links of that kind is already a built-in feature.

Implementation might relate to #41 and #43 which also might potentially extract data without generating the actual blog data (not decided yet).

[novoid]

See comment https://github.com/novoid/lazyblorg/issues/43#issuecomment-715985343 for an implementation approach!