Support configurable indent

[jvehent]

The default indentation in this package is 2 spaces. In some projects, I'd like the ability to use 4 spaces to generate files identical to other YAML parser (Python ruamel.yaml uses 4 spaces). A configurable indentation would be great.

The source hash yaml_emitter_set_indent, but that doesn't seem to be used.

[lelvisl]

It is also necessary to configure which keys to output in a compressed form
Example:
before

- name: John
  age: 30
- name: Mary
  age: 35

after

- {name: John, age: 30}
- {name: Mary, age: 35}

[niemeyer]

This will come with the in-progress v3 work.