BobInit when already initialized should init in pevious recipe directory

[ThomasFeher]

So BobInit should check, if already initialized and if that is the case call BobGoto without arguments and subsequently the init function.

This makes vim-bob re-read all recipes, which is sometimes necessary when updating the recipe repository.

In order to be able to initialize vim-bob for a new recipe directory there should also be something like BobDeInit.