file_path: adds extension-related functions

1. + Extension() for getting extention
2.+ RemoveExtension() to remove extention
3.+ StripExtension(). same as above, but does strip on a copy
4.+ InsertBeforeExtension(). add some string after file name portion but before extension
5.+ AddExtension()
6.+ ReplaceExtension()
7.+ MatchExtension()
file_path: add public comparison functions

1.+ CompareIgnoreCase()
2.+ CompareEqualIgnoreCase()