Allow question marks in plain scalars in flow collections (#105)

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.