Add ? to the list of things to quote when it's an exact value.

[mlwilkerson]

Closes #177

The linked issue includes the full description of the problem this resolves, with reproduction, and notes from the YAML spec. The particularly relevant sections of the spec are:

• 5.3 - the ? character is used as an indicator character, indicating a mapping key
• 7.3.3 - the ? character can only be encoded in Plain Style when it's followed by a non-space "safe" character.

The proposed fix is to add ? to the list of values already being considered for quotation when they appear as exact values.

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Requirements

• Entry in CHANGELOG.md was created
• Link to documentation on https://yaml.org/ is provided in the PR description
• Functionality is covered by newly created tests

[mruoss]

Thanks a lot for the PR and for taking the time to look up and link the documentation. Reading it, made me think we should add tests for all indicator chars if they're on their own. See the comment on the diff.

[mruoss]

Thanks a lot!