Auto-walk until finding a block of a specific type

Having the bot travel the long highways for you is nice and all, but you don't want to miss the signs and landmarks along the way. And if you use `goto sign`, then it just paths back to signs you've already *seen*. This would also be super useful for finding dungeons and such while exploring randomly. I'm assuming this feature hasn't been implemented yet - at least not in a way that isn't overcomplicated or a half-assed solution - because I couldn't find it in the documentation.

Potential implementations:

* A new command, with the syntax looking something like `goto new block_type` or `explorefor block_type`.
* An array of "landmark blocks" that you add to, overriding the goal if the bot discovers any new instances of them.
* Probably a setting called something like `setNewGoalOnLandmark` if you want to just end the pathing instead.
* A setting called something like `exploreForNewBlocks`, which makes the `goto block_type` command look for new blocks instead of old ones.
* The ability to `invert` the `goto block_type` command to go away from explored areas find new blocks.

Also need to have a flag that differentiates between 'was within your render distance' and 'explored'. If Baritone logs a new instance of a block 10 chunks away, I don't know about it. In this instance, both `goto block_type` and `explorefor block_type` or whatever should go to this new block. Though perhaps a couple more commands and settings should be added later for certain use cases:

* A setting called something like `saveLandmarksForLater` that continues exploring for more landmarks, even if it has already found one. But it still doesn't flag it as 'explored', even if the bot paths right by it.
* A command called `goto landmark` - or `goto landmark block_type` for a specific block - that goes to the nearest unexplored landmark.