INTERFACER_LOG should be a path to a file

[densizengin]

I think INTERFACER_LOG should be a path to a file, instead of a path to a directory.

INTERFACER_LOG variable is fetched than passed here like this:

interfacer-proxy/logger/log.go

Line 16 in ec744a3

file, err := os.OpenFile(filepath.Join(logPath, "proxy.log"),

I can take care of this, but tell me what you think.

[densizengin]

It might have a default value of "proxy.log". Such as:

path, ok := os.LookupEnv("INTERFACER_LOG")
if !ok {
	log.Printf(`"INTERFACE_LOG" was not provided; defaulting to "./proxy.log"`)
	path = "./proxy.log"
} else if path == "" {
	return nil, errors.New(`"INTERFACER_LOG" must not be empty"`)
}