Allow question marks in plain scalars in flow collections

[perlpunk]

See http://yaml.org/spec/1.1/#id907281

The question mark isn't mentioned as something special here, only
,[]{}

This commit will allow

[foo?bar]
[foo ? bar]

The PR does only change the behaviour when the question mark is in the middle or at the end of the string, not at the beginning, e.g. [?foo] is handled by a different part of the code.

[perlpunk]

Note: add test to yaml-test-suite:

- [foo?bar]
- [foo ? bar]

[ingydotnet]

@perlpunk Is it easy to allow foo?bar and foo? bar and foo bar?, but not allow foo ? bar or foo ?bar?

The latter ones seem like they would cause more confusion that they'd solve.

[perlpunk]

Is it easy to allow foo?bar and foo? bar and foo bar?, but not allow foo ? bar or foo ?bar?

I think that's not so easy.
And if the ? marker will be removed in flow style in the next YAML version, then this would remove any ambiguity.

[ingydotnet]

Thanks,