Support getting a user's followers

[jdennes]

Similar to #6, but getting a user's followers instead of who they're following.

Can be used like:

package main

import (
	"github.com/jdennes/mubi"
)

func main() {
	api := mubi.NewMubiAPI()
	userId, page, perPage := int64(7995037), 1, 20
	followers := api.GetFollowers(userId, page, perPage)
	// etc
}

Example command implemented:

$ mubi followers --userid 6164674
Atefe - https://mubi.com/users/8819661
Bio: 
Followed on 2021-07-08 14:16:19 +0200 CEST
-----
MAT - https://mubi.com/users/11706732
Bio: mateocc
Followed on 2021-07-08 08:48:59 +0200 CEST
-----
Davi Dantas - https://mubi.com/users/8196670
Bio: A film school dropper. Happy though :)
Followed on 2021-07-07 19:59:34 +0200 CEST
-----
Al - https://mubi.com/users/11031631
Bio: 
Followed on 2021-07-05 10:41:02 +0200 CEST
-----
...