Add `referer` header for mangatown request

[vaskozl]

New downloads fail via mangatown due to a missing referer header. Example commands:

comics-downloader -url="https://www.mangatown.com/manga/spy_x_family/" -last 
comics-downloader -url="https://www.mangatown.com/manga/vagabond/" -last 

Since c9eb756
the referer header is only set for mangakakalot and manganato, but mangatown has a cloudflare rule to block requests on new files without a refer header.

Always setting Referer in pkg/http/client.go fixes this

[Girbons]

thanks for the report @vaskozl!

Always setting Referer in pkg/http/client.go fixes this

I remember that there were cases that always setting this header was causing issues but I'll have a look!