Allow only Custom PAC without GFW List

[neontorrent]

System and Shadowsocksx-NG version:

• macOS 10.13.3 (17D47)
• Shadowsocksx-NG 1.6.1

Expected behavior:
While using Auto Mode, SS uses GFW List along with the User PAC list.
However users outside China does not need GFW List. Emptying the GFW List URL in the Preference does not work and it adds it back automatically.
GFW List URL option should be renamed to something like Custom List, in the Preference and main Manu. If the url textbox is empty, it should be allowed and ignored.

Actual behavior:

N/A For Feature Request

Steps to reproduce:

N/A For Feature Request

How often does this happen?

N/A For Feature Request

ss-local.log

N/A For Feature Request

Application log

N/A For Feature Request

Crash Log

N/A For Feature Request