* Explicitly document key omission in encoding

Currently encoders will omit fields when encoding log entries if the key
configured for that field is set to the empty string. However, this
behavior isn't documented or specified explicitly as being part of the
interface for encoding.

This adds documentation to the `Encoder` interface that explicitly
outlines the behavior that should be implemented when omitting fields.

* Add key for omission to package API

The empty string can be used to omit a key from a log line. However,
there's no explicitly exposed API for using this behavior.

This cleans up the API for users by allowing them to use a constant for
omitting the key instead of throwing around `""` everywhere, or making
their own constant and hoping that `""` will always omit the key.