Make known-unknowns more clear in the reports

[rnjudge]

Describe the Feature In a discussion with @vargenau and his colleague, they mentioned that it would be nice to know known-unknowns in a container. This information is available from Tern but can sometimes be buried in the report. Proposal to make this information more clear.

Use Cases When individuals are inventorying containers it is helpful to understand the limits of Tern's analysis so that users can know what further analysis needs to be done.

Implementation Changes Changes to the output reporting. Perhaps a list of known unknowns in the default report? Also changing the wording in other report formats to make it more clear that further analysis is required. currently Tern reports say: Unknown content included in layer {files}. Please analyze these files separately or Unknown content. Additional analysis may be required

@vargenau thoughts on this? What would be most helpful/clear?