feat: add custom install command parameter (#70)

For example when using Yarn v2 you could do

```yml
- uses: bahmutov/npm-install@v1
  with:
    install-command: yarn --immutable
```

Author: bahmutov

.github/workflows/example-install-command.yml

@@ -0,0 +1,13 @@
+name: example-install-command
+on: [push, pull_request]
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: ./
+        with:
+          install-command: yarn --frozen-lockfile --silent
+          working-directory: examples/yarn
+      - run: npm t
+        working-directory: examples/yarn

README.md

@@ -117,6 +117,18 @@ You can install just the production dependencies (without installing dev depende
     NODE_ENV: production
 ```
 
+### Custom install command
+
+You can use your own install command
+
+```yml
+- uses: bahmutov/npm-install@v1
+  with:
+    install-command: yarn --frozen-lockfile --silent
+```
+
+See [example-install-command.yml](./.github/workflows/example-install-command.yml)
+
 ## NPM
 
 If you are writing your own GitHub Action and would like to use this action as a utility function, import it and run it.

action.yml

@@ -15,3 +15,6 @@ inputs:
   useLockFile:
     description: 'Option to enable or disable use of a lock file (package-lock.json/yarn.lock)'
     required: false
+  install-command:
+    description: 'Custom install command to use'
+    required: false