Expose inspect to allow custom object formatting

[kevinoid]

This PR exposes the inspect method, which is used to format objects for the %o formatting directive, so that it can be called or replaced to suit the needs of users.

Some use cases for this are #185 (inspect with same coloring as debug) and #263 ("better" object formatting). My primary use case is to log objects with a smaller depth to avoid spamming the logs from large/deep objects. Other use cases include using the showHidden and customInspect
options of util.inspect.

Why not just use util.inspect or another custom formatting function directly? My major reasons are:

• It takes more code since every debugged object must be formatted.
• It's not available on browsers, complicating Universal/Isomorphic JavaScript (or interfering with the inspector if polyfilled).
• It adds unnecessary formatting overhead when debugging is disabled.
• It doesn't match debug output colorization without using the undocumented .useColors property.

Note: This PR currently exposes the .inspect property both on debug itself and on the instances. It can be useful for overriding, but if you wanted to minimize the API expansion, it could be exposed as _inspect on debug or otherwise hidden.

Thanks for considering,
Kevin

[thebigredgeek]

Closing in lieu of #327