Preserve the previous $PROMPT_COMMAND status in bash

An attempt to fix #5.

shell/bash.go

@@ -8,7 +8,8 @@ var Bash = Shell(`# Put the line below in ~/.bashrc or ~/bash_profile:
 # The following lines are autogenerated:
 
 __jump_prompt_command() {
-  jump chdir
+  local status=$?
+  jump chdir && return $status
 }
 
 [[ "$PROMPT_COMMAND" =~ __jump_prompt_command ]] || {