pass multiple input files to single --input flag

[skatkov]

solves #14

multiple input files (let's say test.css and test1.css) currently passed to tailwindcss executable each through a separate --input flag.

Like this:

tailwindcss --input test.css --input test2.css --output out.css

But in this case, only last --input flag is being passed on and test.css will not be included in output file.

It would be correct to pass multiple files to a single --input flag. Like so:

tailwindcss --input test.css test2.css --output out.css

I'm not sure if I got it 100% right. It would be sweet if anyone can confirm that this actually resolves the issue.