limited custom_repo option

currently custom repo only support 1 url
```
[[custom_repo]]
name = "chaotic-aur"
url = "https://geo-mirror.chaotic.cx"

# [[custom_repo]]
# name = "blackarch"
# url = "https://at.mirrors.cicku.me/blackarch"
#
[[custom_repo]]
name = "blackarch"
url = "https://mirror.meowsmp.net/blackarch"
```