Proxy Support

[aak1247]

In some area, network is very unstable, so its useful to support proxy.

When I use proxy, I got this error:

 HTTPS_PROXY=socks5://127.0.0.1:1080  HTTP_PROXY=socks5://127.0.0.1:1080 gvm install 1.16

Downloading Go v1.16... Please wait...
Error while downloading https://storage.googleapis.com/golang/go1.16.windows-amd
64.zip - Get https://storage.googleapis.com/golang/go1.16.windows-amd64.zip: htt
p: error connecting to proxy http://socks5://127.0.0.1:1080: dial tcp: lookup so
cks5: getaddrinfow: No such host is known.
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x1 addr=0x0 pc=0x47aa39]

goroutine 1 [running]:
panic(0x779000, 0xc082010080)
        C:/Users/Dan/Desktop/Code/go1.6.2/src/runtime/panic.go:481 +0x3f4
gvm/web.Download(0xc0820101f8, 0x4, 0xc082083d60, 0xd, 0xc0820b6fa0, 0x13, 0x0)
        C:/Users/Dan/Desktop/Scripts/goProjects/src/gvm/web/web.go:28 +0x1539
main.install(0xc0820101f8, 0x4, 0xc082010da5, 0x5, 0xdb82f038184a2a01)
        C:/Users/Dan/Desktop/Scripts/goProjects/src/gvm/main.go:74 +0x455
main.main()
        C:/Users/Dan/Desktop/Scripts/goProjects/src/gvm/main.go:39 +0x7e3