feat: run shell commands asynchronously from command mode

examples/config.rb

@@ -21,7 +21,7 @@ Vamp.configure do |config|
   config.resolve_history_layout = "columns" # or "base_below" for left/right above the base
   # config.popup.command = { anchor: :bottom_left }
   # config.popup.search = { anchor: :center }
-  # Status above, tabs and live battery below; either bar accepts all segments.
+  # Status above, tabs, live battery and pending shell command below.
   config.bar.top = {
     left: [:mode, :path, :diagnostics, :selections, :count, :pending, :buffer],
     center: [], right: [:language, :lsp, :position],
@@ -29,7 +29,7 @@ Vamp.configure do |config|
     position: { padding: 1, leading_edge: "░▓", trailing_edge: "" }
   }
   config.bar.bottom = {
-    left: [:tabs], center: [], right: [:battery],
+    left: [:tabs], center: [], right: [:battery, :shell],
     tabs: {
       title: "{index} {view}{name}{modified}", padding: 1,
       leading_edge: "░▓", trailing_edge: "▓░", first_leading_edge: ""