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.

[bitbybyte]

Will test this later, I'm a little skittish about raising exceptions but I doubt there's a problem here.

[bitbybyte]

Remove this, and if necessary refactor the other use of this constant to pass params, e.g.:

TIMELINES_API = "https://fantia.jp/api/v1/me/timelines/posts"
PAGE_PER_N = 24

response = self.session.get(TIMELINES_API, params={"page": page_number, "per": PAGE_PER_N})

[xWTF]

LGTM, initially I just tried to minimalize changes to code, passing params is definitely better.
I'm a bit busy recently, these requested changes might come in 3-7 days.

[xWTF]

Requested changes have been applied. It's very unlikely that the number 24 will change or anyone would like to change it, so I didn't introduce a new constant.