Override config from environment. See #50

Expects uppercase environment variables of MATTERBRIDGE_PROTOCOL_ACCOUNT_KEY="value"
e.g. you can override this config

[mattermost]
    [mattermost.work]
    Team="yourteam"
    Login="yourlogin"
    Password="yourpass"

by using
MATTERBRIDGE_MATTERMOST_WORK_TEAM="newteam"
MATTERBRIDGE_MATTERMOST_WORK_LOGIN="newlogin"
MATTERBRIDGE_MATTERMOST_WORK_PASSWORD="newpassword"

bridge/bridge.go

@@ -25,6 +25,8 @@ func New(cfg *config.Config, bridge *config.Bridge, c chan config.Message) Bridg
 	accInfo := strings.Split(bridge.Account, ".")
 	protocol := accInfo[0]
 	name := accInfo[1]
+	// override config from environment
+	config.OverrideCfgFromEnv(cfg, protocol, name)
 	switch protocol {
 	case "mattermost":
 		return bmattermost.New(cfg.Mattermost[name], name, c)