Display spaces when showInvisibles is on

I need control over spaces sometimes. It would be nice to add separate option showWhitespaces to toggle space and tab display, because EOL and EOF are badly needed i think.

stringRepeat was improved. See http://jsperf.com/string-repeat-es6/2

I also found that html entities are slower up to 10 times (in Webkit), see http://jsperf.com/non-breakable-spaces-in-dom

Tab character looks bad, I tried to use em dashes with arrows, but these worked bad with some font sizes and faces, so I think it should be displayed using css background. 

I think whitespaces should also use css backgrounds, but wrapping each space into span is not good idea...