Yzis

Implementation of the commands "_n_H", "_n_L" and "_n_M", that move cursor to top, bottom, middle line of screen ( +_n_, -_n_, +_n_).

It manage the end file issue when last line of text aren't the last line of screen.

note : in vim, the argument n_ for the "H" command isn't implemented. As i see no reason, it would be better not to implement it. I did it with middle line of screen +_n lines. If needed i can remove it pretty easily. There's a commented line in mode_command.cpp YModeCommand::middleScreen to make it work as in vim.