Add flow support

Nobody has claimed this yet.

I know this project comes with no dependencies at the moment, but having static typing is very important to me.

Flowtype is a nice fit for this project since `flow-remove-type` is a very simple tool.

I have a fork which adds support for flow: https://github.com/lukejacksonn/servor/compare/master...taylorgoolsby:master

If you still want to keep no dependencies, how do you feel about implementing a hook/plugin system? I have something like this in mind:
```
// Add `transform` as a parameter to `servor`:
module.exports = async ({
root = '.',
module = false,
fallback = module ? 'index.js' : 'index.html',
reload = true,
static = false,
inject = '',
credentials,
port,
transform
} = {}) => {

// sendFile passes files through `transform`:
const sendFile = (res, status, file, ext, encoding = 'binary') => {
if (transform) {
file = transform(file, ext)
}

if (['js', 'css', 'html', 'json', 'xml', 'svg'].includes(ext)) {
res.setHeader('content-encoding', 'gzip');
file = zlib.gzipSync(utf8(file));
encoding = 'utf8';
}
res.writeHead(status, { 'content-type': mimeTypes(ext) });
res.write(file, encoding);
res.end();
};

// flowtype + servor using the transform plugin:
const flowRemoveTypes = require('flow-remove-types');
const servor = require('servor');
const instance = await servor({
root: '.',
fallback: 'index.html',
module: false,
static: false,
reload: false,
inject: '',
credentials: null,
port: 8080,
transform: (file, ext) => {
if (ext === 'js') {
file = flowRemoveTypes(file).toString()
}
return file
}
});
```

Start by reviewing the existing servor request path, especially the sendFile logic shown in the issue, and compare it with the linked fork. Clarify whether the goal is direct Flow support or a general transform hook, then define the supported configuration and verify that transformed JavaScript is served correctly without breaking the dependency-free setup.

Written by the indexing model from the issue text.