Add logics for editing and moving tasks, Update UI for tasks and Implement Import/Export

[jiayushe]

• Implement EditTaskCommand (Fix As a user, I want to modify the due date of tasks #114)
• Implement MoveTaskCommand (Fix As a user, I want to move task from one plan to another #115)
• Implement UI for Task (Fix As a user, I want to view my current progress of tasks within each plan #104)
• Implement ExportCommand and ImportCommand (Fix As a user, I want to import and export my records #8)
• Catch duplicate tasks when adding tasks or moving tasks to plans
• Prettify command text

[coveralls]

Pull Request Test Coverage Report for Build 305

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 499 unchanged lines in 36 files lost coverage.
• Overall coverage decreased (-3.3%) to 36.185%

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Files with Coverage Reduction	New Missed Lines	%
file:/home/travis/build/AY1920S1-CS2103T-W11-1/main/src/main/java/seedu/algobase/model/problem/Author.java	1	73.68%
file:/home/travis/build/AY1920S1-CS2103T-W11-1/main/src/main/java/seedu/algobase/model/problem/Description.java	1	75.0%
file:/home/travis/build/AY1920S1-CS2103T-W11-1/main/src/main/java/seedu/algobase/model/problem/Difficulty.java	1	56.52%
file:/home/travis/build/AY1920S1-CS2103T-W11-1/main/src/main/java/seedu/algobase/model/problem/Remark.java	1	75.0%
file:/home/travis/build/AY1920S1-CS2103T-W11-1/main/src/main/java/seedu/algobase/model/problem/Source.java	1	73.68%
file:/home/travis/build/AY1920S1-CS2103T-W11-1/main/src/main/java/seedu/algobase/model/problem/WebLink.java	1	68.18%
file:/home/travis/build/AY1920S1-CS2103T-W11-1/main/src/main/java/seedu/algobase/commons/util/FileUtil.java	2	83.33%
file:/home/travis/build/AY1920S1-CS2103T-W11-1/main/src/main/java/seedu/algobase/logic/commands/AddCommand.java	2	76.92%
file:/home/travis/build/AY1920S1-CS2103T-W11-1/main/src/main/java/seedu/algobase/logic/commands/HelpCommand.java	2	91.67%
file:/home/travis/build/AY1920S1-CS2103T-W11-1/main/src/main/java/seedu/algobase/logic/commands/DeleteCommand.java	3	58.33%

Totals
Change from base Build 296:	-3.3%
Covered Lines:	1493
Relevant Lines:	4126

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💛 - Coveralls

[le0tan]

Didn't look into the UI part, otherwise looks good to me.

[le0tan]

Is it true that by importing a save file AlgoBase will essentially overwrite the save file at its default location?
Is it better if we change the default save file path at the same time so that the old save file won't be overwritten?