Add `<C-d>`, `<C-u>`, `<C-b>`, `<C-f>`, and other navigational mappings

[ok-nick]

<C-d> - Cursor down half screen <C-u> - Cursor up half screen

<C-b> - Cursor down screen <C-f> - Cursor up screen

H - Cursor top screen M - Cursor middle screen L - Cursor bottom screen

<C-e> - Move screen down one line <C-y> - Move screen up one line

zt - Align top of screen with cursor zz - Align middle of screen with cursor zb - Align bottom of screen with cursor

[kyoheiu]

This app is indeed based on vim-like key mapping, but I have no plans to implement all of vim's spectacular magic. If I ever get frustrated, I will think about these commands.