Allow edit command to delete optional fields

[AbdulrahmanAlRammah]

• Allow deletion of optional fields by edit command
• Update the Testing to match the new functionality
• Update the UG to explain changes

This PR enables users to delete optional fields when editing by entering the prefix without any value.
The add command can now accept entries such as (Add n/John p/12345678 e/) where e/ would just indicate the person doesn't want to input an email. This is worth noting but likely the person won't bother to write the e/ anyways.

Closes #144

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ	Complexity Δ
...in/java/seedu/address/logic/parser/ParserUtil.java	94.44% <100.00%> (-3.71%)	25.00 <0.00> (+3.00)	⬇️

... and 1 file with indirect coverage changes

[liauzhanyi]

lgtm!