Level Filter only works with default output

[nicolaiunrein]

Thank you for this library, however I have the following issue.

The level filter is ignored on output formats other than the default. Am I doing something wrong?

• works fine: cargo run | bunyan --level 30
• shows all levels: cargo run | bunyan --level 30 -o inspect

[dekobon]

I confirmed that this is a bug. All of the formatting output methods such as bunyan, simple, short, and long support the level argument. However, any of the output methods that output JSON, do not seem to honor the --level parameter.