allow extending existing vars

src/ebusd/mqtthandler.cpp

@@ -71,7 +71,8 @@ static const argDef g_mqtt_argDefs[] = {
   {"mqttretain",   O_RETA, nullptr,      0, "Retain all topics instead of only selected global ones"},
   {"mqttqos",      O_PQOS, "QOS",        0, "Set the QoS value for all topics (0-2) [0]"},
   {"mqttint",      O_INTF, "FILE",       0, "Read MQTT integration settings from FILE (no default)"},
-  {"mqttvar",      O_IVAR, "NAME=VALUE[,...]", 0, "Add variable(s) to the read MQTT integration settings"},
+  {"mqttvar",      O_IVAR, "NAME[+]=VALUE[,...]", 0, "Add variable(s) to the read MQTT integration settings "
+   "(append to already existing value with \"NAME+=VALUE\")"},
   {"mqttjson",     O_JSON, "short", af_optional,
    "Publish in JSON format instead of strings, optionally in short (value directly below field key)"},
   {"mqttverbose",  O_VERB, nullptr,      0, "Publish all available attributes"},

src/lib/ebus/stringhelper.cpp

@@ -373,16 +373,14 @@ void StringReplacers::parseLine(const string& line) {
   if (pos == string::npos || pos == 0) {
     return;
   }
-  bool emptyIfMissing = false;
-  string key;
-  if (line[pos-1] == '?') {
-    emptyIfMissing = true;
-    key = line.substr(0, pos-1);
-  } else {
-    key = line.substr(0, pos);
-  }
+  bool emptyIfMissing = line[pos-1] == '?';
+  bool append = !emptyIfMissing && line[pos-1] == '+';
+  string key = line.substr(0, (emptyIfMissing || append) ? pos-1 : pos);
   FileReader::trim(&key);
   string value = line.substr(pos+1);
+  if (append) {
+    value = get(key).str() + value;
+  }
   FileReader::trim(&value);
   if (value.find('%') == string::npos) {
     set(key, value);  // constant value