Add --cached flag to update command

[swissspidy]

Description As the title says, this change adds support for a --cached flag to the update command.

Fixes #79.

How has this been tested? Manual testing + unit test coverage.

Types of changes Extends UpdateCommand and Runner::run() to skip downloading the remote repository and instead use the local one.

Checklist:

• [x] My code is tested.
• [x] My code follows the WordPress code style.
• [x] My code has proper inline documentation.

[codecov-io]

Codecov Report

Merging #94 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff           @@
##             master   #94   +/-   ##
======================================
  Coverage        75%   75%           
- Complexity      334   336    +2     
======================================
  Files            23    23           
  Lines           804   804           
======================================
  Hits            603   603           
  Misses          201   201

Impacted Files	Coverage Δ	Complexity Δ
inc/Runner.php	100% <100%> (ø)	9 <5> (+2)	:arrow_up:

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[swissspidy]

Right, I thought it did return null. Well then of course we have to check whether the directory exists.

[swissspidy]

Looks like the docblock was removed when I resolved the most recent merge conflicts. Added back now.