Add line_callback and edit async helpers

helpers.lua

@@ -112,7 +112,7 @@ end
 function helpers.async(cmd, callback)
     return spawn.easy_async(cmd,
     function (stdout, stderr, reason, exit_code)
-        callback(stdout)
+        callback(stdout, exit_code)
     end)
 end
 
@@ -120,10 +120,19 @@ end
 function helpers.async_with_shell(cmd, callback)
     return spawn.easy_async_with_shell(cmd,
     function (stdout, stderr, reason, exit_code)
-        callback(stdout)
+        callback(stdout, exit_code)
     end)
 end
 
+-- run a command and execute a function on its output line by line
+function helpers.line_callback(cmd, callback)
+    return spawn.with_line_callback(cmd, {
+        stdout = function (line)
+            callback(line)
+        end,
+    })
+end
+
 -- }}}
 
 -- {{{ A map utility