Add a `--screenshot` argument that implies `--headless` and is used to take a screenshot of a page from the command line.
Default is a full page screenshot, but `--window-size=width[,height]` can change this.
A path for the screenshot can be supplied with `--screenshot=/path/to/file`.
MozReview-Commit-ID: 13tUjk2Yrsl
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extra : rebase_source : d26ed0856af882ad52a606941685b2c91577d3bf
