Turn --preserve-quotes on by default

[Samathy]

I ran yamlfmt with it's default settings over a codebase with a lot of YAML config.
It removes quotation marks from strings by default. This changes the semantic meaning of the files being formatted in the following ways:

Booleans:
"yes" and "no" are strings, but without quotes, they are parsed as booleans. Docker-compose in particular wants strings and chokes when given booleans.

Integers:
"10" is a string, but without quotes it is parsed as a number. This can cause problems.

This formatter should only be moving text around, not changing the meaning of it.
The --preserve-quotes flag should be on by default to avoid these issues.

[iranzo]

Had this same issue with Ansible playbooks, they got completely messed up, when enabled, apparently everything worked fine

[hofbi]

I also think it would make sense to have this by default.

[danielhollas]

Sorry to pile on, but I was hit by this many times (and just right now). Would be good to have a safer default.