Add check that URLs don't contain Windows path separators (`\`)

[ai]

On widows file paths will include \ instead of / and source-map doesn’t convert slashes, but source map will work only with UNIX slashes.

We can fix it in app, but I think, that we need to fix it here, because every source-map node.js user must to repeat this fix.

Autoprefixer issue: nDmitry/grunt-autoprefixer#25

I can helps with PR, but how we should do this? Try to load path (it wil be in node.js and browserfy) and if path.sep == "\\" replace all \ to /?

[fitzgen]

We can't rely on the path module because it only exists in node, not the web or inside firefox.

The reason we use / is because a source map's sources are URLs and URLs use /.

I'm not against supporting alternative path separators as long as it doesn't negatively affect the existing 99% use case.

[ai]

I mean, that if library user will set file to ..\build\main.css, source-map shoudl replace it to ../build/main.css in output map.

[ai]

We can try to load path and if it will be loaded and will say, that path.sep == "\\" we will know, that we are in Windows’s node.js.

[sokra]

This library takes URLs as input. A path is not a valid input if it's separated with \. In my opinion it's the job of the application to pass a valid URL.

Instead we should validate if the passed string is a valid URL.

[ai]

@sokra OK, no problem, I make filter in my app.

But we really need validation, because with issue is in many apps. For example, Rework also send file name, not URL: https://github.com/reworkcss/css-stringify/blob/master/lib/source-map-support.js#L22