open_dir always uses Explorer.exe - Use python os.startfile() instead to invoke custom Dir Handlers

Many Windows users have Explorer replacements such as Total Commander, XYPlorer, Directory Opus, xplorer2, etc. These usually replace explorer.exe via changing the Folder handler in the registry at: [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Folder\shell\]

However - even when someone has a custom folder handler setup - SublimeText currently always opens a folder via Explorer.exe and not through an os-specific way. An alternative to fix this that I have tested is to use os.startfile("C:\folderpath"). For example the following will respect the user's custom directory handler:

import sublime, sublime_plugin, os

class OpenContainingDirCommand(sublime_plugin.TextCommand):

    def run(self, edit):

        branch, leaf = os.path.split(self.view.file_name())

        os.startfile(branch)

So please use os.startfile(dirpath) for things such as "Browse Package".