Add --skip-update-check flag to plugin list and theme list commands

[Nikschavan]

By default WP-CLI does a force update check when running 'wp plugin list' and 'wp theme list'
'--skip-update-check' flag can be added to these commands which can bypass the force udpate check

Fixes #299

[Nikschavan]

@schlessera The tests are failing on PHP 5.6 because of null coalescing operator in the one-time-login plugin which is being downloaded in one of the tests.

Is there anything that should be done to fix this? maybe changing the tests to download another dummy repo with a simple plugin. This failing test seems to checking that the branch name is removed from the project downloaded from GitHub (in a separate PR).

Also, I am not sure if I can add any new tests for this new flag introduced int this PR, since the stdout will be same?
I am new to behat testing.

[swissspidy]

One Time Login apparently requires PHP 7.1 nowadays. So yeah we should download a different one.

[schlessera]

This also needs a Behat test to ensure the flag works (and keeps working across future updates).

[Nikschavan]

Converting the PR to draft until i debug the failing tests.

[Nikschavan]

This is now ready for a review. The failing PHP 5.6 tests should be fixed with #302

[schlessera]

Thanks for the PR, @Nikschavan !

[Nikschavan]

@schlessera - Is there any (rough) timeline when this can be available in the nightly/stable of wp-cli, I believe until a release is created on an individual command, it is not available in the nightly of wp-cli as well, correct?

I also created wp-cli/wp-cli-bundle#354 to discuss if there is a way to make nightlies include nightly code changes of individual commands as well.