Ensure that we fallback to the first argument when -i is missing

[RobinMalfait]

If you have a command like this:

npx tailwindcss app.css -o output.css

Then you would get a deprecation warning:

[deprecation] Running tailwindcss without -i, please provide an input file.

However, this is a deprecation warning, but the CLI won't work if you don't have a -i. For now we will fallback to the first argument (in this case app.css) as the input.