Extended changelog, group by tags

[rafinskipg]

Add the possibility to group by tags on the extended log just like here https://github.com/angular/angular.js/blob/master/CHANGELOG.md

Use this kind of commit messages to grep tag names :

chore(release): v1.4.0 codename(jaracimrman-existence)

[rafinskipg]

#32

[richardthombs]

I love the idea of grouping by tag, but would prefer that the grep pattern be made optional. Ie, if the commit message or tag annotation matches the grep, then use it to decorate the tag, but otherwise just use the tag name.