Use export LP_MARK_PREFIX="\n" to seperate the prompt into two lines

I like to separate bash prompt into two lines. One line is to show the status; another is the space for user to type in.

`LP_MARK_PREFIX` is default to "", but user can `export LP_MARK_PREFIX="\n"` to make the prompt like this:

```
[dryman:~]
 $ cd project/liquidprompt/
```