Implement PriorityCommandParser class and PriorityCommand class

Codecov Report

Attention: Patch coverage is 73.46939% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../seedu/address/logic/commands/PriorityCommand.java	59.25%	9 Missing and 2 partials :warning:
.../seedu/address/logic/parser/AddressBookParser.java	0.00%	1 Missing :warning:
...main/java/seedu/address/model/person/Priority.java	0.00%	1 Missing :warning:

Files with missing lines	Coverage Δ	Complexity Δ
...ain/java/seedu/address/logic/parser/CliSyntax.java	`90.00% <100.00%> (+1.11%)`	`1.00 <0.00> (ø)`
...in/java/seedu/address/logic/parser/ParserUtil.java	`87.93% <100.00%> (+4.91%)`	`20.00 <2.00> (+3.00)`
...du/address/logic/parser/PriorityCommandParser.java	`100.00% <100.00%> (ø)`	`4.00 <4.00> (?)`
.../seedu/address/logic/parser/AddressBookParser.java	`85.71% <0.00%> (-4.29%)`	`11.00 <0.00> (ø)`
...main/java/seedu/address/model/person/Priority.java	`75.00% <0.00%> (-6.82%)`	`7.00 <0.00> (ø)`
.../seedu/address/logic/commands/PriorityCommand.java	`59.25% <59.25%> (ø)`	`8.00 <8.00> (?)`