Need rules to enforce string quoting for keys

[HenryGessau]

The current quoted-strings rule does not check keys.

Consider:

---
one: 1
'two': 2
"three": 3
["a", b]: 4
{'a': a, b: 'b'}: 5

There is currently no rule to enforce consistent quoting for key strings.

The quoted-strings rule can be extended with an option to also check keys. However, there may be a need to have different quoting rules for key strings and value strings.