Add support for en ("–") and em dashes ("—") as separator between version and date

[koppor]

I very much like to use an en dash to separate version and date. I know that typography demands an em dash (see e.g., https://www.quora.com/Would-it-be-correct-to-use-an-en-dash-to-separate-section-number-and-title-2-3-Forest-Wildlife-or-would-an-em-dash-be-more-appropriate), but that dash was too large for me.

I also added support for em dash.

On the one hand, a CHANGELOG should be easily parsable. On the other hand, it should look nice for end users. A changelog should be more read than created. Thus, the toolings for handling might have more effort - than causing typography irritation at the readers.

[koppor]

Maybe, I should add a command-line option?

[charphi]

The specification doesn't require a specific dash so ... why not ?
There is already a discussion on that topic at olivierlacan/keep-a-changelog#497