Allow embedders to decorate ranges in the buffer

Creating this to centralize discussion on @mofux's idea to support monaco-like decorations as discussed in https://github.com/xtermjs/xterm.js/issues/1380#issuecomment-379848832 and https://github.com/xtermjs/xterm.js/issues/791#issuecomment-448372516.