discriminate between en-dash, em-dash

[nschloe]

Typically, en-dashes are created by double dashes --, em-dashes by triple dashes --- (e.g., compose key syntax or LaTeX). This pull request implements this behavior.

[AaronO]

@nschloe Thanks for the PR ! It looks great overall. And it makes sense to distinguish both.

However could you fix the tests ?

1. Update test/tests/text.smartypants.html to use en dashes rather than em dashes.
2. Add some em dashes both in the markdown and html

(You can run tests with npm test).

[AaronO]

@nschloe Any update on this ?