Add four functions to open new lines

    +---------------------+-----------+
    |Function             |Key map    |
    +---------------------+-----------+
    |open-line-and-indent |C-o        |
    +---------------------+-----------+
    |open-line-below      |C-return   |
    +---------------------+-----------+
    |open-line-above      |C-S-return |
    +---------------------+-----------+
    |new-line-in-between  |s-return   |
    +---------------------+-----------+