[ProblemSet] Refactor stat command.
[ProblemSet] Refactor stat command.

* '-t': show statistics by tag.
* '-g': fixes graph display bug.

Signed-off-by: Eric Wang <7593b1d387b45602cb19d471721471f6115519cf@gmail.com>

heshamsafi/leetcode-cli,skygragon/leetcode-cli,skygragon/leetcode-cli,heshamsafi/leetcode-cli