Add flexible tmux configuration load function.

Add function `_get_user_tmux_conf` to helper script `plugin_functions`.
Function is searching for the users tmux configuration on multiple
places by a prioritized order.
The response is used within`_tmux_conf_contents` to read in the content
as normally.
Add new environment variable `TMUX_PLUGIN_MANAGER_CONFIG_LOCATION` which
is optional to be defined.
If so it has the highest priority to be loaded, despite if the file
exist or not.
XDG directory support has been added as well by the second priority
location at `$XDG_CONFIG_HOME/tmux/tmux.conf`.

scripts/helpers/plugin_functions.sh

@@ -15,8 +15,30 @@ _tpm_path() {
 
 _CACHED_TPM_PATH="$(_tpm_path)"
 
+# Get the absolute path to the users configuration file of TMux.
+# This includes a prioritized search on different locations.
+#
+_get_user_tmux_conf() {
+  # Define the different possible locations.
+  custom_location="$TMUX_PLUGIN_MANAGER_CONFIG_LOCATION"
+  xdg_location="$XDG_CONFIG_HOME/tmux/tmux.conf"
+  default_location="$HOME/.tmux.conf"
+
+  # Search for the correct configuration file by priority.
+  if [ -n "$custom_location" ]; then
+    echo "$custom_location"
+
+  elif [ -f "$xdg_location" ]; then
+    echo "$xdg_location"
+
+  else
+    echo "$default_location"
+  fi
+}
+
 _tmux_conf_contents() {
-	cat /etc/tmux.conf ~/.tmux.conf 2>/dev/null
+  user_config=$(_get_user_tmux_conf)
+	cat /etc/tmux.conf "$user_config" 2>/dev/null
 	if [ "$1" == "full" ]; then # also output content from sourced files
 		local file
 		for file in $(_sourced_files); do