Add option to fail if snapshot files are missing

To be used in CI - if we don't commit the snapshots, we're not testing anything.

Fixes #87

README.md

@@ -114,6 +114,10 @@ Run Cypress with `--env updateSnapshots=true` in order to update the base image
 
 Run Cypress with `--env failOnSnapshotDiff=false` in order to prevent test failures when an image diff does not pass.
 
+### Requiring snapshots to be present
+
+Run Cypress with `--env requireSnapshots=true` in order to fail if snapshots are missing. This is useful in continuous integration where snapshots should be present in advance.
+
 ### Reporter
 
 Run Cypress with `--reporter cypress-image-snapshot/reporter` in order to report snapshot diffs in your test results. This can be helpful to use with `--env failOnSnapshotDiff=false` in order to quickly view all failing snapshots and their diffs.

src/command.js

@@ -9,6 +9,7 @@ import { MATCH, RECORD } from './constants';
 
 const screenshotsFolder = Cypress.config('screenshotsFolder');
 const updateSnapshots = Cypress.env('updateSnapshots') || false;
+const requireSnapshots = Cypress.env('requireSnapshots') || false;
 const failOnSnapshotDiff =
   typeof Cypress.env('failOnSnapshotDiff') === 'undefined';
 
@@ -78,6 +79,10 @@ export function matchImageSnapshotCommand(defaultOptions) {
           diffPixelCount,
           diffOutputPath,
         }) => {
+          if (added && requireSnapshots && !failOnSnapshotDiff) {
+            throw new Error(`New snapshot ${name} was added, but 'requireSnapshots' was set to true.
+            This is likely because this test was run in a CI environment in which snapshots should already be committed.`);
+          }
           if (!pass && !added && !updated) {
             const message = getErrorMessage(
               diffSize,