Basic functionality
Issues:
- at the moment in IE10, IE11 and Edge selection highlighting (visual feedback) doesn't work as expected
- because of source code block-buffering on some pages, selecting "all" (CTRL+A) may cause selection to cover only part of the code. It's propably because during selection "user-select: none" is applied to body instead to each unselectable element
- IE and Edge - ctrl+A doesn't work