Store RGB color information in cterm attribute entries.

This is a temporary hack: When terminal dependencies are removed, only RGB will
be used to describe colors(terminal-specific UIs will translate RGB to terminal
color codes)
API: Events: Implement the 'redraw:update_line' event
API: Events: Implement the 'redraw:status_line' event
API: Events: Implement the 'redraw:ruler' event
API: Events: Implement the 'redraw:cursor' event
API: Implement vim_redraw_screen to force redraw events
API: Implement vim_command_output function

This function can be used by API clients to execute a command and capture the
output.