Filter paid fanclubs from timeline if -p is provided

[xWTF]

Motivation

Some creators post low-res images on free tier, which will be downloaded when using -n flag (I'm using this to fetch new posts automatically) and mess up the library.

Being able to filter out non-paid posts when downloading from timeline addresses the above issue.

Implementation

Added a check to append free_plan=not_free when -p is specified with -n. Since in the original implementation -n won't have any effect when -p specified, this should not change any intended behavior.

Behavior changes:

• -n -p: Download latest paid posts
• -f -n: Throw a misuse error
• -f -p: Throw a misuse error
• -n or -p or -f: Same as previous implementation

[bitbybyte]

Since in the original implementation -n won't have any effect when -p specified, this should not change any intended behavior.

I think it's fine for this PR if no behavior changes, but it'd probably be nice to do this eventually.