Expanded yes command and fixed case-sensitive issues

[mhdsyfq77]

• Can now type in "y"/"ye"/"yes" to confirm delete/clear
• Commands are now case-insensitive

Closes #153 and closes #160

[codecov-io]

Codecov Report

Merging #169 into master will increase coverage by 0.08%. The diff coverage is 57.14%.

@@             Coverage Diff              @@
##             master     #169      +/-   ##
============================================
+ Coverage     75.53%   75.62%   +0.08%     
  Complexity      565      565              
============================================
  Files            93       93              
  Lines          1713     1715       +2     
  Branches        174      173       -1     
============================================
+ Hits           1294     1297       +3     
  Misses          338      338              
+ Partials         81       80       -1     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...rc/main/java/seedu/address/logic/LogicManager.java	43.47% <50.00%> (+4.84%)	5.00 <0.00> (ø)
...java/seedu/address/logic/parser/GradPadParser.java	75.00% <100.00%> (ø)	14.00 <0.00> (ø)

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