Add before_inline_transition_to callback

[svanhesteren]

Currently we can only specify an after callback. But sometimes you want to perfom some actions before the state is transitioned.
Atm we do not have this possibility so a before_inline_transition_to would be handy to have.

[julik]

That would be nice - but should that "before..." also be able to abort the transition? I didn't want to decide on that so that's why I haven't implemented it initially.

[svanhesteren]

I think yes. Atm I need it to be in a transaction block that rolls back when something fails. But maybe you can pass in an option.

[julik]

Well, if it raises it will anyway abort whatever is happening. But should it work like a callback - and be able to abort the transaction by returning a falsey value from the block? I would prefer to avoid that as creating an implicit block return with a falsy value can easily make the entire action "silently" abort, which is not ideal. Maybe we should actually state that "no, you can't manage the transition from the block except for when your block raises"?