Add half screen scroll keybindings

[mkatychev]

• Fixed keys j/k behaving in reverse
• Added half screen scrolling with behaviour similar to vim/less; from vim docs:

                            *CTRL-U*
CTRL-U          Scroll window Upwards in the buffer.  The number of
            lines comes from the 'scroll' option (default: half a
            screen).  If [count] given, first set the 'scroll'
            option to [count].  The cursor is moved the same
            number of lines up in the file (if possible; when
            lines wrap and when hitting the end of the file there
            may be a difference).  When the cursor is on the first
            line of the buffer nothing happens and a beep is
            produced.  See also 'startofline' option.
            {difference from vi: Vim scrolls 'scroll' screen
            lines, instead of file lines; makes a difference when
            lines wrap}

                            *CTRL-D*
CTRL-D          Scroll window Downwards in the buffer.  The number of
            lines comes from the 'scroll' option (default: half a
            screen).  If [count] given, first set 'scroll' option
            to [count].  The cursor is moved the same number of
            lines down in the file (if possible; when lines wrap
            and when hitting the end of the file there may be a
            difference).  When the cursor is on the last line of
            the buffer nothing happens and a beep is produced.
            See also 'startofline' option.
            {difference from vi: Vim scrolls 'scroll' screen
            lines, instead of file lines; makes a difference when
            lines wrap}

[AMythicDev]

Thanks for fixing those bugs. I really appreciate it