feat: ability to use user provided tslint config

This change adds the ability to use tslint.conf provided by the user.
If the user doesn't have a config file, we fall back to the default
gts config file.

src/lint.ts

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+import * as fs from 'fs';
 import * as path from 'path';
 import {Configuration, Linter} from 'tslint';
 import * as ts from 'typescript';
@@ -27,11 +28,13 @@ import {Options} from './cli';
  */
 export function lint(
     options: Options, files: string[] = [], fix = false): boolean {
-  const tslintConfigPath = path.join(options.gtsRootDir, 'tslint.json');
+  const configPath =
+      fs.existsSync(path.join(options.targetRootDir, 'tslint.json')) ?
+      path.join(options.targetRootDir, 'tslint.json') :
+      path.join(options.gtsRootDir, 'tslint.json');
 
   const program = createProgram(options);
-  const configuration =
-      Configuration.findConfiguration(tslintConfigPath, '').results;
+  const configuration = Configuration.findConfiguration(configPath, '').results;
   const linter = new Linter({fix, formatter: 'codeFrame'}, program);
   const srcFiles = files.length > 0 ? files : Linter.getFileNames(program);
   srcFiles.forEach(file => {

test/test-lint.ts

@@ -42,7 +42,6 @@ test.serial('createProgram should return an object', async t => {
   });
 });
 
-
 test.serial('lint should return true on good code', async t => {
   await withFixtures(
       {
@@ -148,4 +147,31 @@ test.serial('lint should not throw for unrecognized files', async t => {
       });
 });
 
+test.serial('lint should prefer user config file over default', async t => {
+  const CUSTOM_LINT_CODE = 'const t: Object;';
+
+  // By defualt the above should fail lint.
+  await withFixtures(
+      {
+        'tsconfig.json': JSON.stringify({files: ['a.ts']}),
+        'a.ts': CUSTOM_LINT_CODE
+      },
+      async () => {
+        const okay = lint.lint(OPTIONS);
+        t.is(okay, false);
+      });
+
+  // User should be able to override the default config.
+  await withFixtures(
+      {
+        'tsconfig.json': JSON.stringify({files: ['a.ts']}),
+        'tslint.json': JSON.stringify({}),
+        'a.ts': CUSTOM_LINT_CODE
+      },
+      async () => {
+        const okay = lint.lint(OPTIONS);
+        t.is(okay, true);
+      });
+});
+
 // TODO: test for when tsconfig.json is missing.