(Team 1) Implement a "Span" class

[chenlica]

For @rajesh9625 @sandeepreddy602 @inkudo

It's used to capture a substring in a document.

Check if this concept is already available in other packages, include Lucene and System T. Talk to Team 6 (@FlavioBayer) and @zuozhi about System T if needed.

This concept is critical for other tasks.

Let me and @JavierJia be the reviewers.

[sandeepreddy602]

The issue is completed in the pull request #59