Allows setting content type header via middleware

[sguter90]

Currently the "Content-Type" header is automatically set to the value returned by mime.getType.
If the "Content-Type" header was already set by a middleware then this header will be overwritten.

Within this change the "Content-Type" header is only set if it is not already defined.
The getFileContentType method is added to split Content-Type detection from rendering logic.
If the Content-Type can't be detected, then no header is sent.

An example middleware which I use for a website which I migrated from PHP to 11ty:

function (req, res, next) {
        if (/.*\.php$/.test(req.url)) {
          res.setHeader('Content-Type', 'text/html; charset=utf-8');
        }

        next();
}

[zachleat]

I think I’m on board here but it looks like you checked in the sample middleware to cli.js by accident, no? That seems like test code

[zachleat]

Shipping with v1.0.4!