Added multiple file/folder selection support (closed yasirkula#10)

Note that the Scripting API has also changed slightly with this new feature. The OnSuccess callback now returns string[] instead of string and dialog functions now take an allowMultiSelection parameter (false by default)

Added OpenUPM instructions

Fixed yasirkula#18

Fixed yasirkula#18

- Fixed yasirkula#16

- Fixed a possible NullPointerException on Android when selecting a new folder using SAF

Updated README