Introduce new 'info' warning level.

There are some messages that may be relevant to developers when looking at the log from assetgraph, but are not a fatal event. This introduces a new event `info` which implementations can hook into.

The `info` event enables certain messages to be sent without implementations that break on `warn` stopping when the event is sent. An example of this is `buildProduction --stoponwarning`.

The changeset here changes the `splitCssIfIeLimitIsReached` transform to emit `info` instead of `warn`, since the nature of the message is a hint for the developer to improve the code base, but the built code base still works as intended.

This change should of course be followed by an event listener for `info` events in assetgraph-builder. And we might also want to evaluate the error levels in the rest of the transforms to see if their level can be degraded.