Add escape_all_strings option to Emitter

[paulhauner]

First of all, thanks for this library. It's been great and has got me out of some sticky situations where serde is not flexible enough. Much appreciated.

This PR makes a change to ensure typing between loading and emitting.

Problem

Consider the following YAML:

example: "0x00"

After one round of YamlLoader -> YamlEmitter the YAML is now:

example: 0x00

After a second round the YAML is then:

example: 0

The YAML has now changed significantly and information has been lost.

Solution

I added an emitter.escape_all_strings(bool) function which forces all YAML::String nodes to be wrapped in double-quotes.

I would be interested on your opinion on two points:

• I would have used single-quotes, however it seemed much simpler to use double-quotes. I am not a YAML expert and I would be interested to hear of any side-effects.
• The key is now wrapped in double-quotes too (e.g., "example": "0x00"). I am not sure if this has side-effects too.

Thank you for you time, happy to make changes as you require.