add post install hooks

[adrelanos]

As a distribution (and possibly for others) it's useful to run some post install scripts after installing Tor Browser. Examples:

• We create a ~/Downloads folder.
• We disable Tor Browser's internal updater, because it's not as secure yet.
• Possibly in future more such as changing browser homepage.

Could you add a feature please to execute a /etc/tl/post-install (or so) script please?