Generate license text when missing for detected license

[jonabc]

closes #189

This PR takes an alternate approach to reducing friction when license data is only found in a package manager file. Rather than writing the package manager content to the cached data, this takes an approach of caching the default license text, as known and provided by licensee.

The generated data is only included if no other license contents are found (from LICENSE files, readme text, etc) and a non-"other" license is found for a dependency.

This means that for the (hopefully) majority of cases where license text isn't included, the tool will be able to provide some default text. The only complication is that many package managers don't include publish dates or authors as part of the package metadata, which makes generating any copyright clauses hard. As a result I am stripping any copyright contents from the license templates when caching data.

I've set the sources property for the text to specify that the text is auto generated, rather than coming from a file.

@mlinksva is this approach viable? I'm not sure if this would be ok, given that the license content isn't coming from the package author and doesn't include a copyright.