VeinMiner Mod allows you to mine an entire vein of blocks at once.
You can enable it by holding a pickaxe, axe or shovel and clicking the V button that pops up in the bottom-right corner.
If you mine one of a certain type of block, all the connected blocks of the same type will also break (up to 128 blocks at a time).
Do note that your tool will take damage for every block that is broken. (Due to causing a lost of enchantments, this is no longer the case in 0.12, until there’s a workaround)