Use String#includes instead of String#indexOf

[andria-dev]

Fixes #336

In all instances where String#indexOf was used to determine if a substring existed, I replaced the calls with String#includes.

Affected files:

• live-server.js
• test/cli.js
• test/acceptance.js
• injected.html
• index.js

All tests passed locally with all the changes in this PR 👍