Improve xolox#misc#path#equals() for case-insensitive filesystem

[matteoquintiliani]

At the moment the function xolox#misc#path#equals() is defined to use the case-insensitive path comparison only for Windows system. Actually, also on Mac OS X you can have a case-insensitive filesystem.

I will provide a patch which use an optional global variable g:xolox#misc#case_insensitive_filesystem which can override the default values.