Unable to specify content src in config.xml

It looks like the latest `1.2.0` update has hardcoded the filename to use when loading the webview as `index.html`. It should respect the content src setting in the main config file.

```xml

```

Here is the link to the relevant cordova documentation:
https://cordova.apache.org/docs/en/latest/config_ref/index.html#content

Here is a link to the possible culprit in the code:
https://github.com/ionic-team/cordova-plugin-ionic-webview/commit/ab22a992150e0f500224cd75b01a2d7ec25088ce#diff-7044543564ea722137708c081ff83c04R374