Disallow argument for all one-word commands

[florenciamartina]

Fixes #166

[codecov-io]

Codecov Report

Merging #180 into master will increase coverage by 0.26%.
The diff coverage is 91.48%.

@@             Coverage Diff              @@
##             master     #180      +/-   ##
============================================
+ Coverage     72.72%   72.98%   +0.26%     
- Complexity      745      753       +8     
============================================
  Files           110      109       -1     
  Lines          2130     2158      +28     
  Branches        262      271       +9     
============================================
+ Hits           1549     1575      +26     
  Misses          495      495              
- Partials         86       88       +2     

Impacted Files	Coverage Δ	Complexity Δ
...va/seedu/address/logic/commands/CclearCommand.java	100.00% <ø> (ø)	3.00 <0.00> (ø)
...ava/seedu/address/logic/commands/ClistCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...java/seedu/address/logic/commands/ExitCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...java/seedu/address/logic/commands/HelpCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...java/seedu/address/logic/commands/ListCommand.java	83.33% <ø> (ø)	3.00 <0.00> (ø)
...va/seedu/address/logic/commands/MclearCommand.java	100.00% <ø> (ø)	3.00 <0.00> (ø)
...ava/seedu/address/logic/commands/MlistCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...ava/seedu/address/logic/commands/ResetCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...edu/address/logic/commands/UnassignallCommand.java	80.00% <ø> (ø)	3.00 <0.00> (ø)
...edu/address/logic/parser/OneWordCommandParser.java	91.30% <91.30%> (ø)	20.00 <20.00> (?)
... and 2 more

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