add before_install and before_uninstall

[rolandwalker]

This is intended as a tool to force some truly odd cases like #2654. (The container is weird, we need to pre-preprocess it).

I intentionally left this undocumented for now in case we think of a better way to do this, for example as an extension to #2649.

[vmrob]

I think this sounds like a great feature, though out of curiosity, with respect to the oddities of the aforementioned dmg, why not just attempt opening it read-only and then use the default as a fallback option?

[rolandwalker]

Because I'm assuming two things

1. @phinze had a good reason for only allowing read-only mounts
2. there will be other odd cases where we want the before_install hammer

[phinze]

Yeah +1 @rolandwalker I'd rather capture the oddity of this particular dmg inside the Cask definition rather than adding complexity to the code run for every Cask.

The idea for these sort of blocks is that we can capture edge cases and then take stock - if we find ourselves doing a pattern many times, we can promote it to the official DSL or the standard code path.