Refactor logging control, adding logLevel option

[eemeli]

Make the logging controllable via a new option:

logLevel: 'silent' | 'error' | 'warn' | 'debug'

The default is 'warn', and acts pretty much as previously. At least for now 'debug' doesn't add anything on top of it.

Setting logLevel: 'error' silences warnings during YAML.parse() and doc.toJS().

Setting logLevel: 'silent' also does its best to silence even parsing errors, resulting in output that should represent at least a part of the original input, even if it isn't entirely valid YAML.

As a related change, the warning about casting collections to string when used a map key & mapAsMap is not enabled is now emitted only during the toJS() call, rather than being added to the doc.warnings array.