documented --user-agent switch (cf #275)

cli/single-file

@@ -131,6 +131,8 @@ const args = require("yargs")
 	.boolean("remove-alternative-images")
 	.options("save-raw-page", { description: "Save the original page without interpreting it into the browser (puppeteer, webdriver-gecko, webdriver-chromium)" })
 	.boolean("save-raw-page")
+	.options("user-agent", { description: "User-agent of the browser (puppeteer, webdriver-gecko, webdriver-chromium)" })
+	.string("user-agent")
 	.options("web-driver-executable-path", { description: "Path to Selenium WebDriver executable (webdriver-gecko, webdriver-chromium)" })
 	.string("web-driver-executable-path")
 	.argv;