[FEATURE] Allow warming up all available sites at once

[eliashaeussler]

This PR extends the existing --sites command option by a special keyword all. When used, all available sites are warmed up.

Resolves: #717

[codeclimate[bot]]

Code Climate has analyzed commit d4ed7bdc and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 90.9% (0.0% change).

View more on Code Climate.

[coveralls]

Pull Request Test Coverage Report for Build 11251261859

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 90.972%

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Totals
Change from base Build 11250857327:	0.03%
Covered Lines:	1048
Relevant Lines:	1152

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💛 - Coveralls