Renewal of plugin

This is basically a rewrite of the plugin, adding the following features:
  - Allows to specify the command to launch using an array instead of a string. It gives better compatibility with Windows, and less quotes to write.
  - Instead of using only {0} as placeholder, adds the {filepath}, {dirname} and {basename} placeholders.
  - Always uses the user-specified command to open folders instead of the system one.
  - Support for two distinct commands to launch when opening directly a folder, or when opening a file's containing folder.
  - Support for compressed installation in ST3
  - Support for being installed with any package name, not only "Open Folder"
  - Usage of the recommended way subprocess.popen to launch the command, preventing Sublime Text of hanging while the file manager is open (a problem mainly in Windows)
  - Drop-in replacement, if user has an old Open Folder settings file, it will work just exactly as before.