Use a Reasonable User Agent

[solonovamax]

Please make sure that the text "rom1504/img2dataset" is always included in the user agent header.

currently it is not.

this is a major issue, especially since it does not respect robots.txt.
if someone wants to block it, the lack of identifiable user agent is a huge issue.

See: https://mas.to/@alexskunz/113315215546762650

[solonovamax]

This is fixed by #300.

In the same vein of "actually respecting people hosting websites", please also merge #312, #302, and #301.