Add new remark command

[Rachael-Chan]

closes #115

Added new remark command which allows users to add remarks to existing contacts. UI is also updated to show remarks on contact list.

• [x] Added Tests
• [x] Updated user documentation

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 89.87342% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/main/java/seedu/address/ui/PersonCard.java	0.00%	3 Missing :warning:
...c/main/java/seedu/address/model/person/Person.java	80.00%	1 Missing and 1 partial :warning:
...c/main/java/seedu/address/model/person/Remark.java	77.77%	1 Missing and 1 partial :warning:
.../java/seedu/address/storage/JsonAdaptedPerson.java	83.33%	0 Missing and 1 partial :warning:

Files with missing lines	Coverage Δ	Complexity Δ
src/main/java/seedu/address/logic/Messages.java	95.45% <100.00%> (+0.58%)	17.00 <2.00> (+2.00)
...java/seedu/address/logic/commands/EditCommand.java	97.72% <100.00%> (+0.02%)	13.00 <0.00> (ø)
...va/seedu/address/logic/commands/RemarkCommand.java	100.00% <100.00%> (ø)	11.00 <11.00> (?)
...a/seedu/address/logic/parser/AddCommandParser.java	100.00% <100.00%> (ø)	5.00 <0.00> (ø)
.../seedu/address/logic/parser/AddressBookParser.java	100.00% <100.00%> (ø)	14.00 <0.00> (+1.00)
...ain/java/seedu/address/logic/parser/CliSyntax.java	88.88% <100.00%> (+1.38%)	1.00 <0.00> (ø)
...eedu/address/logic/parser/RemarkCommandParser.java	100.00% <100.00%> (ø)	2.00 <2.00> (?)
.../java/seedu/address/model/util/SampleDataUtil.java	96.77% <100.00%> (ø)	4.00 <0.00> (ø)
.../java/seedu/address/storage/JsonAdaptedPerson.java	94.80% <83.33%> (-1.03%)	21.00 <0.00> (+1.00)	:arrow_down:
...c/main/java/seedu/address/model/person/Person.java	95.45% <80.00%> (-1.99%)	30.00 <2.00> (+2.00)	:arrow_down:
... and 2 more