Implement exercise T4L2/tags-add

[jovnc]

Exercise Review

Exercise Discussion

#61

Checklist

• If you require a new remote repository on the Git-Mastery organization, have you created a request for it?
• Have you written unit tests using repo-smith to validate the exercise grading scheme?
• Have you tested the download script using test-download.sh?
• Have you verified that this exercise does not already exist or is not currently in review?
• Did you introduce a new grading mechanism that should belong to git-autograder?
• Did you introduce a new dependency that should belong to app?

[jovnc]

@MousseDm @woojiahao Opened new PR to help continue implementation of #85

Here are some changes made:

1. add repo-smith test
2. improve code quality
3. update README.md to follow guidelines
4. improve verification logic

There are some major differences in the verification logic, to ensure that we verify according to the task steps (ensure task 1 is completed before task 2) rather than verifying all at once and returning all comments to users.