Fix file/path loading on Windows

[ghost]

When browsing to a file on the server, e.g. http://10.1.1.76:1338/stream/Y:/_watch/some-file.mkv, I get the following error:

`Error: ENOENT, no such file or directory 'C:\Y:_watch\some-file.mkv'
at Object.fs.statSync (fs.js:689:18)
at module.exports (C:\Users\Adam\Documents\mkvcast\lib\controllers\stream-file.js:6:22)
at Layer.handle [as handle_request] (C:\Users\Adam\Documents\mkvcast\node_modules\express\lib\router\layer.js:76:5)
at next (C:\Users\Adam\Documents\mkvcast\node_modules\express\lib\router\route.js:100:13)
at Route.dispatch (C:\Users\Adam\Documents\mkvcast\node_modules\express\lib\router\route.js:81:3)
at Layer.handle [as handle_request] (C:\Users\Adam\Documents\mkvcast\node_modules\express\lib\router\layer.js:76:5)
at C:\Users\Adam\Documents\mkvcast\node_modules\express\lib\router\index.js:227:24
at Function.proto.process_params (C:\Users\Adam\Documents\mkvcast\node_modules\express\lib\router\index.js:305:12)
at C:\Users\Adam\Documents\mkvcast\node_modules\express\lib\router\index.js:221:12
at Function.match_layer (C:\Users\Adam\Documents\mkvcast\node_modules\express\lib\router\index.js:288:3)``

Note that the file is in Y:\ but the application is running out of C: and for some reason trying to look up the file there.