Fixed material lookup issues, added placement.

You can now add a placement-source under the item-source for your sprout
in sprouts.yml to specify what block the seed must have to be planted in
the world.

This allows cool things such as having a crop that only grows in
cobblestone. Bear in mind, we cannot stop the Minecraft Server from
uprooting the base block so choose a compatible base block within
MoreMaterials (or whereever your custom item is coming from).

We find Rose to be the most compatibile.
Signed-off-by: Chris Sanders <zidane@outlook.com>