Modify HelpCommand to take keyword of command

[HynRidge]

Add HelpCommandParser to parse user argument
Add MESSAGE_USAGE in ListCommand,RedoCommand,UndoCommand,ExitCommand,ClearCommand.
Add unit test for HelpCommand and HelpCommandParser

Note: Safe to merge!!

[codecov-io]

Codecov Report

Merging #186 into master will decrease coverage by 0.28%.
The diff coverage is 54.83%.

@@             Coverage Diff              @@
##             master     #186      +/-   ##
============================================
- Coverage     63.88%   63.60%   -0.29%     
- Complexity      482      487       +5     
============================================
  Files            94       95       +1     
  Lines          1595     1621      +26     
  Branches        150      151       +1     
============================================
+ Hits           1019     1031      +12     
- Misses          523      534      +11     
- Partials         53       56       +3     

Impacted Files	Coverage Δ	Complexity Δ
...ava/seedu/address/logic/commands/ClearCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...va/seedu/address/logic/commands/CommandResult.java	88.23% <ø> (ø)	10.00 <0.00> (ø)
...java/seedu/address/logic/commands/ExitCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...java/seedu/address/logic/commands/FindCommand.java	100.00% <ø> (ø)	6.00 <0.00> (ø)
...java/seedu/address/logic/commands/ListCommand.java	80.00% <ø> (ø)	3.00 <0.00> (ø)
...java/seedu/address/logic/commands/OpenCommand.java	55.55% <ø> (ø)	8.00 <0.00> (ø)
...java/seedu/address/logic/commands/RedoCommand.java	100.00% <ø> (ø)	4.00 <0.00> (ø)
...ava/seedu/address/logic/commands/RetagCommand.java	88.00% <ø> (ø)	9.00 <0.00> (ø)
...java/seedu/address/logic/commands/ShowCommand.java	100.00% <ø> (ø)	8.00 <0.00> (ø)
.../java/seedu/address/logic/commands/TagCommand.java	73.07% <ø> (ø)	9.00 <0.00> (ø)
... and 8 more

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[luo-git]

Code is quite clean. I like it. LGTM!

Edit: Thanks for cleaning up the help message format too.