Add tag-related features

[zekone]

Features added:

• Add tags to a contact
  eg. add tag -id 1 -t frontend -t java
• Delete tags from a contact
  eg. delete tag -id 1 -t frontend -t java

Documentation update:

• UG: Added guides for the two newly added features above.
• UG: Added notes about ... for items that can be used multiple time such as -t.
• DG: Updated use cases for add and delete tag.

Others:

• Added tests for the new features.

[codecov]

Codecov Report

Merging #63 (36e01b7) into master (05c2290) will decrease coverage by 2.94%.
Report is 9 commits behind head on master.
The diff coverage is 63.93%.

@@             Coverage Diff              @@
##             master      #63      +/-   ##
============================================
- Coverage     77.75%   74.82%   -2.94%     
- Complexity      627      702      +75     
============================================
  Files           103      112       +9     
  Lines          1951     2232     +281     
  Branches        200      257      +57     
============================================
+ Hits           1517     1670     +153     
- Misses          377      468      +91     
- Partials         57       94      +37     

Files	Coverage Δ
...in/java/seedu/address/commons/util/StringUtil.java	95.65% <100.00%> (+0.91%)	⬆️
src/main/java/seedu/address/logic/Messages.java	68.57% <ø> (ø)
...seedu/address/logic/commands/AddPersonCommand.java	100.00% <ø> (ø)
...edu/address/logic/commands/DeleteEventCommand.java	100.00% <100.00%> (ø)
...a/seedu/address/logic/parser/AddCommandParser.java	77.77% <100.00%> (+2.77%)	⬆️
...ain/java/seedu/address/logic/parser/CliSyntax.java	94.44% <100.00%> (+0.32%)	⬆️
...eedu/address/logic/parser/DeleteCommandParser.java	77.77% <100.00%> (+2.77%)	⬆️
.../seedu/address/logic/parser/HelpCommandParser.java	100.00% <ø> (ø)
.../seedu/address/logic/parser/ListCommandParser.java	75.00% <100.00%> (ø)
src/main/java/seedu/address/model/AddressBook.java	61.66% <100.00%> (-5.00%)	⬇️
... and 22 more

... and 3 files with indirect coverage changes

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[Nixx162]

LGTM