Include 'end-of-document' indicator ("...") in YAML exporter

[nichtich]

Some YAML readers expect this, see http://yaml.org/spec/1.2/spec.html#id2760395

Current output

$ catmandu convert Mock --size 2 to YAML

---
n: 0

---
n: 1

expected output

---
n: 0
...

---
n: 1
...

See https://gist.github.com/nichtich/97f06bbfa249f5d33e2d#file-doi2csl-md for a use case.

[vpeil]

fixed with release 0.9207