[feature request] Limit channel downloads to dates between x and y

[Cyber1000]

May overlap a little bit with #113.

Problem:

• Use case 1: I want to download a channel, but not from the beginning, but lets say only the last 3 months (and not older videos).
• Use case 2: I already downloaded a channel, just want to download from where I left off until today.

Proposal: add two date-fields (from and to) and download only links where its upload date is between those dates.