Fix tests on windows for using `illuminate/filesystem` and add option for absolute paths

[florisbosch]

Related: #174

This pull request fixes tests on windows for using illuminate/filesystem and adds an option for absolute paths.

The fix for windows was to check for existing files before trying to use the filesize or lastmodified functions.

I've added the option to enable/disable absolute paths. It is working on my machine but I don't think this is the most optimal or elegant way to solve this problem. With this solution the current features are backwards compatible without any changes in current implementations.

My idea for handling this problem in the future is to allow for the combination of multiple disks, this way we can use multiple local or cloud folders in the same viewer. Using absolute paths is usually not the most secure way of handling files especially when you are not fully in control of the hosting of your Laravel application.