Enable programmatic installation (#33)

1. Ensure `path` aalways reflects the state of the filesystem.
    Previously it would be set once when the module is loaded and not
    updated if the installation is added or removed from the filesystem
2. Allow optional installation on `npm install`
    It may be easier for some projects to include this as a dependency
    if the chromium installation can be made conditional.

Author: ricksbrown

README.MD

@@ -143,6 +143,24 @@ export NODE_CHROMIUM_CACHE_DISABLE=true
 # node_chromium_cache_disable=true
 ```
 
+### Skip Automatic Chromium Install
+
+Chromium will ordinarily be installed when you exectute `npm install` however you may wish to skip this step if you are going to defer installation and perform it programatically at a later stage. Below is an example of how to do so.
+
+```bash
+export NODE_CHROMIUM_SKIP_INSTALL=true
+
+# or in .npmrc like so:
+# node_chromium_skip_install=true
+```
+
+Then install it programatically when you need it:
+
+```js
+chromium.install().then(function() {
+    // do stuff...
+});
+```
 ## Contributors
 <table>
   <tr style="background: #ffec86">

index.js

@@ -15,6 +15,12 @@ function getBinaryPath() {
 }
 
 module.exports = {
-    path: getBinaryPath(),
+    /*
+     * The path property needs to use a getter because the binaries may not be present for any number of reasons.
+     * Using a getter allows this property to update itself as needed and reflect the current state of the filesystem.
+     */
+    get path() {
+        return getBinaryPath();
+    },
     install: require('./install')
 };

install.js

@@ -142,7 +142,11 @@ async function install() {
 
 if (require.main === module) {
     // Module called directly, not via "require", so execute install...
-    install();
+    if (config.getEnvVar('NODE_CHROMIUM_SKIP_INSTALL').toLowerCase() === 'true') {
+        console.info('Skipping chromium install');
+    } else {
+        install();
+    }
 }
 
 tmp.setGracefulCleanup(); // Ensure temporary files are cleaned up when process exits

test/test-install.js

@@ -11,6 +11,7 @@ const testUtils = require('./_utils');
 const utils = require('../utils');
 const config = require('../config');
 const install = require('../install');
+const chromium = require('..');
 
 test.before(t => {
     // Deleting output folder
@@ -36,13 +37,15 @@ test.serial('Canary Test', t => {
 test.serial('Before Install Process', t => {
     const binPath = utils.getOsChromiumBinPath();
     t.false(fs.existsSync(binPath), `Chromium binary is found in: [${binPath}]`);
+    t.falsy(chromium.path, 'chromium.path should not be defined when there is no installation');
 });
 
 test.serial('Chromium Install', async t => {
     await install();
     const binPath = utils.getOsChromiumBinPath();
     const isExists = fs.existsSync(binPath);
     t.true(isExists, `Chromium binary is not found in: [${binPath}]`);
+    t.true(fs.existsSync(chromium.path), 'chromium.path should be defined after installation');
 });
 
 test.serial('Different OS support', async t => {