Supporting environment config

[paulhill]

Supporting environment config
#100

simply added this config for Viper

        replacer := strings.NewReplacer(".", "_")
	viper.SetEnvKeyReplacer(replacer)
	viper.SetEnvPrefix("STELLAR")
	viper.AutomaticEnv()

and updated Viper to the v1.0.0 label

Bridge server now supports env overrides of nested config properties according to the Viper conventions.
e.g.

export STELLAR_ACCOUNTS_RECEIVING_ACCOUNT_ID=GBLZJANPTTBW3R4E2XJM5NZGRPNTH6AJSGOTWWAU7SYEC3UN7QWTP5OH

[paulhill]

I'm a Go n00b so if I ballzed up the update process I might need some guidance on how to do it properly.