Allow Custom Domain for Shortened URLs

[jaiyankargupta]

The extension currently uses chrome.runtime.getURL('') to generate shortened URLs. Allow users to specify a custom domain for their shortened URLs.

Steps to Reproduce:

1. Install the extension.
2. Attempt to configure a custom domain for shortened URLs.
3. Observe that it is not possible with the current implementation.

Expected Behavior:
Users should be able to configure a custom domain for their shortened URLs through the extension's options page.