Add changed file information to snow log

[belkarx]

Description

Adds info on whether a file has been added/modified in snow log via a --changed-information flag

The output is in JSON, and a status field is added to each TreeFile output in each commit with "added" or "modified" (creating a "deleted" flag proved to be a bit ambiguous in definition and I believe required changes to the underlying commit diffing function [I may be wrong here and just not familiar enough with TS/this codebase to make it work, but either way it didn't happen]).

Fixes #144

Checks

• [ x ] I added my name to the contributor list in CONTRIBUTORS.md
• [ x ] I understand that my work will be licensed under the given license of SnowFS
• [ x ] I am the original author, or I have authorization to submit this work.

Type of change

Please delete options that are not relevant.

• [ x ] New feature (non-breaking change which adds functionality)
• [ x ] This change requires a documentation update

[sebastianrath]

@belkarx Thanks so much for the PR. Please give me a few days to review and test this. I'm back in the office next week.