Addition of themed gravels and corresponding gravel ores

[IronPiston]

In a similar vein to the themed cobblestone, gravel variants for each species of stone would make world generation look much more unified. It could still function as normal gravel, dropping flint and responding to gravity, but would match the surrounding stone. As it is, the grey gravel stands out against some of the more colorful stone types.

Expanding upon this, gravel ores similar to Tinkers' Construct would also be a neat addition. It could be a toggle-able feature that would give incentive to make shovels out of stronger materials, and also add diversity to the normal mining of stone with a pickaxe. These gravel ores would also texture themselves to the species of gravel they were embedded in.

[kyeiti]

I don't think I'll get around to this very soon. (I will stick to problems for now)

If anyone feels like adding this (or parts of this), please comment here to let others know and create a pull request once it's done.