improve goto command and make it possible to omit the keyword "goto"
* make it possible to give goto a number prepended with a minus or plus sign to jump that many lines back/forward in the buffer
* don't give an error message if a number greater than the number of lines in the file is given. instead jump to the last line. 'goto 0' will go to line 1.
* make it possible to omit the 'goto' keyword. in other words: the command '123' will go to line 123. '-10' will go 10 lines up and so on.