feat: add custom shell icon option #4

Adds a `@tmux-nerd-font-window-name-shell-icon` tmux option to overwrite the shell icon.

README.md

@@ -44,6 +44,13 @@ set -g window-status-current-format '#[fg=magenta]#W'
 set -g window-status-format         '#[fg=gray]#W'
 ```
 
+### Custom shell icon
+
+To specify a custom shell icon, use the following option to set any icon you prefer:
+
+```sh
+set -g @tmux-nerd-font-window-name-shell-icon ""
+```
 ## How it works
 
 When installed, your window names will automatically update to a Nerd Font that matches the activity (ex: vim, bash, node, ect...).

bin/tmux-nerd-font-window-name

@@ -2,13 +2,28 @@
 
 NAME=$1
 
+# Get shell icon, using user-provided option if available
+function get_shell_icon(){
+  local default_shell_icon=""
+  local shell_icon="$(tmux show -gqv '@tmux-nerd-font-window-name-shell-icon')"
+
+  # Alternate shell icon is nonempty
+  if [ -n "$shell_icon" ]; then
+    echo "$shell_icon"
+  else
+    echo "$default_shell_icon"
+  fi
+}
+
+SHELL_ICON=$(get_shell_icon)
+
 get_icon() {
 	case $NAME in
 	tmux)
 		echo ""
 		;;
 	fish | zsh | bash | tcsh)
-		echo ""
+		echo $SHELL_ICON
 		;;
 	vi | vim | nvim | lvim)
 		echo ""