feat: add baseUrl App option

The option can be used to override hardcoded defaults from @octokit/endpoint.

So that we can now use it to work on GitHub Enterprise installations:

```js
const app = new App({
  id: APP_ID,
  privateKey: PRIVATE_KEY,
  baseUrl: 'https://github-enterprise.com/api/v3'
})
```

README.md

@@ -89,6 +89,18 @@ const app = new App({
 })
 ```
 
+## Using with GitHub Enterprise
+
+The `baseUrl` option can be used to override default GitHub's `https://api.github.com`:
+
+```js
+const app = new App({
+  id: APP_ID,
+  privateKey: PRIVATE_KEY,
+  baseUrl: 'https://github-enterprise.com/api/v3'
+}) 
+```
+
 ## License
 
 [MIT](LICENSE)

index.js

@@ -6,11 +6,11 @@ const getCache = require('./lib/get-cache')
 const getInstallationAccessToken = require('./lib/get-installation-access-token')
 const getSignedJsonWebToken = require('./lib/get-signed-json-web-token')
 
-function App ({ id, privateKey, cache }) {
+function App ({ id, privateKey, baseUrl, cache }) {
   const state = {
     id,
     privateKey,
-    request,
+    request: baseUrl ? request.defaults({ baseUrl }) : request,
     cache: cache || getCache()
   }
   const api = {

test/test.js

@@ -169,4 +169,24 @@ describe('app.js', () => {
         expect(options.cache.set.callCount).to.equal(1)
       })
   })
+
+  it('supports custom base url', () => {
+    nock('https://github-enterprise.com/api/v3')
+      .post('/app/installations/123/access_tokens')
+      .reply(201, {
+        token: 'foo'
+      })
+
+    const options = {
+      id: APP_ID,
+      privateKey: PRIVATE_KEY,
+      baseUrl: 'https://github-enterprise.com/api/v3'
+    }
+    const appWithCustomEndpointDefaults = new App(options)
+
+    return appWithCustomEndpointDefaults.getInstallationAccessToken({ installationId: 123 })
+      .then(token => {
+        expect(token).to.equal('foo')
+      })
+  })
 })