fix(dropdown): correct TS typings

ref: https://stackoverflow.com/q/45716754/1418796

Closes #1754

src/dropdown/dropdown-config.spec.ts

@@ -7,4 +7,15 @@ describe('ngb-dropdown-config', () => {
     expect(config.up).toBe(false);
     expect(config.autoClose).toBe(true);
   });
+
+  it('should allow setting "inside" and "outside" value for autoClose', () => {
+    const config = new NgbDropdownConfig();
+
+    // This test looks like having trivial assertions but its goal
+    // is to prove that we've got TS typings right.
+    config.autoClose = 'outside';
+    expect(config.autoClose).toBe('outside');
+    config.autoClose = 'inside';
+    expect(config.autoClose).toBe('inside');
+  });
 });

src/dropdown/dropdown-config.ts

@@ -8,5 +8,5 @@ import {Injectable, TemplateRef} from '@angular/core';
 @Injectable()
 export class NgbDropdownConfig {
   up = false;
-  autoClose = true;
+  autoClose: boolean | 'outside' | 'inside' = true;
 }