Implement mark and unmark for tasks

[nobodyishappy]

The MarkTask Command

The marktask command mark tasks as done. The command has the following format:

marktask TASK_INDEX

Parameter: TASK_INDEX

• Must be a numeric string representing a number between 1 and total number of tasks.
• If the value is not acceptable, the error message The task index provided is invalid is displayed.

If the task is already marked, the command is still accepted and succeeds.

Things to note

• marktask does not remove the task from the person.

The UnmarkTask Command

The unmarktask command mark tasks as undone. The command has the following format:

unmarktask TASK_INDEX

Parameter: TASK_INDEX

• Must be a numeric string representing a number between 1 and total number of tasks.
• If the value is not acceptable, the error message The task index provided is invalid is displayed.

If the task is already unmarked, the command is still accepted and succeeds.

This PR is linked to #6.

[codecov]

Codecov Report

Attention: Patch coverage is 82.79570% with 32 lines in your changes are missing coverage. Please review.

Project coverage is 76.31%. Comparing base (7cf9ff9) to head (9584609).
Report is 17 commits behind head on master.

Files	Patch %	Lines
...main/java/seedu/address/model/task/TaskStatus.java	54.54%	8 Missing and 2 partials ⚠️
.../java/seedu/address/model/util/SampleDataUtil.java	16.66%	5 Missing ⚠️
...in/java/seedu/address/logic/parser/ParserUtil.java	60.00%	2 Missing and 2 partials ⚠️
...java/seedu/address/model/task/TaskDescription.java	71.42%	2 Missing and 2 partials ⚠️
...c/main/java/seedu/address/model/task/TaskName.java	71.42%	2 Missing and 2 partials ⚠️
src/main/java/seedu/address/ui/TaskCard.java	0.00%	3 Missing ⚠️
src/main/java/seedu/address/ui/PersonCard.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #40      +/-   ##
============================================
+ Coverage     76.03%   76.31%   +0.28%     
- Complexity      495      549      +54     
============================================
  Files            82       92      +10     
  Lines          1565     1744     +179     
  Branches        141      161      +20     
============================================
+ Hits           1190     1331     +141     
- Misses          342      374      +32     
- Partials         33       39       +6     

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[breezetall]

Looks great to me!

[Kaya3842]

The code is neat and clean. Approved!

[chin-herng]

LGTM so far! Do modify the UI to display whether a task is done.

[Yskie]

Looks good to me!

[breezetall]

I like that you have created a util class to better facilitate our testing. The code looks good!