Modify EditContactCommand to allow deleting of telegram field

[jonasngs]

This PR fixes #591

This PR address the issue of users being unable to delete a telegram field of a contact despite it being an optional field.

The following classes have been modified:

1. EditContactDescriptor: Store an additional boolean field to indicate if users wish to delete the telegram field of a contact
2. EditContactParser: handle the case when no argument for the te/ prefix was provided
3. EditContactCommand: handle the execution of the correct command

Additionally, the user guide was updated to inform users of this feature.

[codecov-io]

Codecov Report

Merging #592 into master will increase coverage by 13.79%.
The diff coverage is 28.31%.

@@              Coverage Diff              @@
##             master     #592       +/-   ##
=============================================
+ Coverage     18.89%   32.68%   +13.79%     
- Complexity      370      756      +386     
=============================================
  Files           227      245       +18     
  Lines          4392     5020      +628     
  Branches        524      619       +95     
=============================================
+ Hits            830     1641      +811     
+ Misses         3487     3203      -284     
- Partials         75      176      +101     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...rc/main/java/seedu/address/logic/LogicManager.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...c/main/java/seedu/address/logic/ParserManager.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...eedu/address/logic/commands/ViewCommandResult.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
.../commands/modulelistcommands/AddModuleCommand.java	31.81% <ø> (ø)	3.00 <0.00> (ø)
...mmands/modulelistcommands/CalculateCapCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...modulelistcommands/TargetCapCalculatorCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...modulelistcommands/ViewArchivedModulesCommand.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/ContactListParser.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
... and 157 more

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[murtubak]

LGTM.