Implement tagging feature for Contact

[jonasngs]

Fixes #382

This PR adds a tagging feature for Contact and modifies the related classes that are involved in tagging.

The following classes were modified:

1. AddContactCommand: Change usage message
2. AddContactParser: Modify parse method to parse tags
3. EditContactCommand: Account for Tag when creating a new Contact
4. EditContactParser: Modify parse method to parse tags
5. EditContactDescriptor: Add methods to set and get Tag
6. Contact: Create a field for Tag
7. JsonAdaptedContact: Modify toModelType() method to convert a JsonAdaptedTag to a Tag object
8. ContactCard: Modify constructor to display tags on GUI

[codecov-io]

Codecov Report

Merging #383 into master will decrease coverage by 0.25%.
The diff coverage is 7.14%.

@@             Coverage Diff              @@
##             master     #383      +/-   ##
============================================
- Coverage     25.31%   25.06%   -0.26%     
  Complexity      339      339              
============================================
  Files           176      176              
  Lines          2864     2897      +33     
  Branches        332      338       +6     
============================================
+ Hits            725      726       +1     
- Misses         2074     2106      +32     
  Partials         65       65              

Impacted Files	Coverage Δ	Complexity Δ
...ommands/contactlistcommands/AddContactCommand.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...mmands/contactlistcommands/EditContactCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...nds/contactlistcommands/EditContactDescriptor.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...ic/parser/contactlistparsers/AddContactParser.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...c/parser/contactlistparsers/EditContactParser.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../java/seedu/address/model/util/SampleDataUtil.java	21.42% <0.00%> (-16.08%)	1.00 <0.00> (ø)
...java/seedu/address/storage/JsonAdaptedContact.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
src/main/java/seedu/address/ui/ContactCard.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...main/java/seedu/address/model/contact/Contact.java	64.86% <100.00%> (+0.97%)	15.00 <0.00> (ø)

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[michael-setia]

LGTM aside from possible incomplete input checking.