When providing `--css-modules insertHashHere` microbundle will treat all CSS files as CSS modules

Microbundle supports providing a custom hash generator name for `CSS modules` by providing `--css-modules insertHashPatternHere` (eg.: `--css-modules _[hash:base64:5]`.

### The Bug

But then Microbundle will also treat all CSS files as CSS modules, instead of just changing the generated signature of the actual CSS modules.

By the documentation, all CSS files should be considered a CSS module if a flag `--css-modules true` is provided.

### Possibly source of the Bug

The bug seems to originate on https://github.com/developit/microbundle/blob/b1a637486234a2ae784ccf0c512321e2d3efef7c/src/lib/css-modules.js#L8 where it only considers the "should only suffix modules" when the option `null` is being passed.

### The caveats

I suppose the `--css-modules` flag should only support `null`, `true` and `false` (Default Behaviour (Only when suffix), Enforce AutoCSS modules, Disable CSS modules completely).

Since there is the flag `--no-css-modules` there should be one for specifying the hash too. For example `--css-modules-hash _[hash:base64:5]`.

This would also avoid confusion since it is not documented in the README that using a custom hash would enforce CSS modules.

I would be more than happy to provide a patch 😄