Enhancement: Support If-Modified-Since

[tempusfrangit]

PGET should support if-modified-since logic

This should be opt-in via cli-option. This might be duplicate of #61

[MacOS]

Is anyone working on this? If no, I'm happy to take that over.

[tempusfrangit]

It currently does not have anyone working on it.

There was some internal discussion about using mtime or ctime of the local file to create the IMS check. I don't think that discussion was captured in the issue.

There is also a question how the range-request stuff ends up working with IMS. I haven't looked into the standard for that.

[MacOS]

In short, there has been already a lot of discussion about it that is not written down, and no one has come up with an implementation to argue about. This seems to me, as if I should not invest in it - am I right @tempusfrangit?