Implemented support for repository defined node version files such as '.nvmrc'

src/main.ts

@@ -1,6 +1,7 @@
 import * as core from '@actions/core';
 import * as installer from './installer';
 import * as auth from './authutil';
+import fs = require('fs');
 import * as path from 'path';
 import {URL} from 'url';
 
@@ -15,6 +16,16 @@ export async function run() {
       version = core.getInput('version');
     }
 
+    if (!version) {
+      const versionFile = core.getInput('node-version-file');
+
+      if (!!versionFile) {
+        const versionFilePath = path.join(__dirname, '..', versionFile);
+        version = fs.readFileSync(versionFilePath, 'utf8');
+        core.info(`Resolved ${versionFile} as ${version}`);
+      }
+    }
+
     if (version) {
       let token = core.getInput('token');
       let auth = !token || isGhes() ? undefined : `token ${token}`;