feat(install): support npm config for chromium revision (#2290)

This patch:
- adds `npm_config_puppeteer_chromium_revision` support
- clarifies api documentation about variables from npm config

docs/api.md

@@ -266,7 +266,8 @@ The Puppeteer API is hierarchical and mirrors the browser structure. On the foll
 
 ### Environment Variables
 
-Puppeteer looks for certain [environment variables](https://en.wikipedia.org/wiki/Environment_variable) to aid its operations. These variables can either be set in the environment or in the [npm config](https://docs.npmjs.com/cli/config).
+Puppeteer looks for certain [environment variables](https://en.wikipedia.org/wiki/Environment_variable) to aid its operations.
+If puppeteer doesn't find them in environment, lowercased variant of these variables will be used from the [npm config](https://docs.npmjs.com/cli/config).
 
 - `HTTP_PROXY`, `HTTPS_PROXY`, `NO_PROXY` - defines HTTP proxy settings that are used to download and run Chromium.
 - `PUPPETEER_SKIP_CHROMIUM_DOWNLOAD` - do not download bundled Chromium during installation step.

install.js

@@ -30,7 +30,9 @@ const downloadHost = process.env.PUPPETEER_DOWNLOAD_HOST || process.env.npm_conf
 const puppeteer = require('./index');
 const browserFetcher = puppeteer.createBrowserFetcher({ host: downloadHost });
 
-const revision = process.env.PUPPETEER_CHROMIUM_REVISION || require('./package.json').puppeteer.chromium_revision;
+const revision = process.env.PUPPETEER_CHROMIUM_REVISION || process.env.npm_config_puppeteer_chromium_revision
+  || require('./package.json').puppeteer.chromium_revision;
+
 const revisionInfo = browserFetcher.revisionInfo(revision);
 
 // Do nothing if the revision is already downloaded.