[Feature request] Export only lowest frequency value

[BigBoyBarney]

Hi! Thanks for the app!

I would like to order my cards based on their frequency, which requires a simple numeric value in a sort field.

Currently, the only way to export frequencies is by exporting them as a bulleted list using the {frequencies} marker. Would it be possible to add a marker {frequency-minimum} or something, that only exports the lowest frequency, without dictionary name?

So in the case of the example above, instead of:

• Anime & J-Drama: 1673
• JPDB: 536
• JPDB: 10437 カ

simply export:
536

Thanks!

[ripose-jp]

When I get around to implementing the extended marker syntax from #143, I'll do this.