Change default enabled behavior of Log class

[Pessimistress]

• If log is disabled, all levels of logging are expected to be blocked. Currently it doesn't block L0 including warnings, errors and level=0 logs. The enabled flag is practically meaningless (can just be replaced with log.level = 0).
• It's reasonable to expect that new Log().log('hello')() would actually log something to the console. Currently log instances are disabled by default.
• It's cumbersome for the users to have to do log.enable(true).setLevel(1) just to see some logs.

In both deck.gl and luma.gl, the log objects are enabled on initialization, e.g. https://github.com/uber/deck.gl/blob/7.3-release/modules/core/src/utils/log.js#L3
Therefore changing the default here does not affect the default behavior in our primary use cases.

[coveralls]

Coverage increased (+4.3%) to 45.593% when pulling e76b55b on x/default-level into 4bd5096 on master.

[ibgreen]

LGTM.

Maybe something can be done to document the changes?