Support cached plagiarism analysis

[tcibinan]

Currently if one launches just the same analysis several times it will be submitted to MOSS or JPlag every time even if nothing has changed since. Probably it can be nice to have optional support for cached analyses.

Basically it means that if no source files have changed and no new source files have appeared since some previous analysis then the underlying MOSS or JPlag analysis can be skipped and the corresponding previous analysis results can be returned.

Talking about the implementation I'm thinking about calculating analysis checksums and using it as an analysis unique identifier.

[nikita715]

@tcibinan, hi! Thank you for the issue. It's a good idea, I will add this functionality in the next release.