discriminate between en-dash, em-dash

Typically, en-dashes are created by double dashes `--`, em-dashes by triple dashes `---` (e.g., [compose key syntax](http://en.wikipedia.org/wiki/Wikipedia:How_to_make_dashes) or [LaTeX](http://tex.stackexchange.com/a/3821/13262)). This pull request implements this behavior.