Implement Deletetag Command

[wang-xinrong]

#81

[codecov]

Codecov Report

Attention: Patch coverage is 84.00000% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 77.73%. Comparing base (2cfee46) to head (767e207).

Files	Patch %	Lines
...seedu/address/logic/commands/DeletetagCommand.java	74.19%	2 Missing and 6 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #82      +/-   ##
============================================
+ Coverage     77.53%   77.73%   +0.19%     
- Complexity      511      523      +12     
============================================
  Files            82       84       +2     
  Lines          1589     1639      +50     
  Branches        156      161       +5     
============================================
+ Hits           1232     1274      +42     
- Misses          313      315       +2     
- Partials         44       50       +6     

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[yyccbb]

LGTM