Codec | Implement a codec version similar to json

[KevinBLT]

We also were in demand of writing yaml (config) files.

This first commit is trying to implement a version that is similar to the json codec.

See test/codec_test.dart for references.

Fixes #113

Example usage:

const Map test = {
  'string' : 'Test text.',
  'number' : 1337,
  'bool'   : false,
  'list'   : [1,2,3,4],
  'map'    : {
    'a' : 1,
    'b' : 2,
    'c' : 'text',
    'd' : ['a', 'b', 'c']
  }
};

print(yaml.encode(test));

/* Output:
---
string: 'Test text.'
number: 1337
bool: false
list:
  - 1
  - 2
  - 3
  - 4
map:
  a: 1
  b: 2
  c: 'text'
  d:
    - 'a'
    - 'b'
    - 'c'
*/

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[KevinBLT]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[jonasfj]

For encoding and modifying YAML, see package:yaml_edit.

You can use it to write YAML by creating an empty document and updating it.

But in general when rendering YAML, you'll want to preserve comments and formatting which is what yaml_edit does.

[KevinBLT]

I agree with the preserving of comments etc. - and I didn't know the yaml_edit package so far. Nonetheless, I still think it would at least be a "nice to have" to expose it as a codec, too, giving developers unified functionalities for such tasks.