Support colormap in foliage textures

[deathcap]

Currently, ProgrammerArt colors grass and leaves green, directly in the texture file.

MC uses a colormap so it can colorize foliage differently in other biomes. The texture files themselves include a grayish color instead, as a placeholder. ProgrammerArt might want to adopt this convention, as well (preferably still retaining the pre-greenified textures for compatibility, as they are easier to support directly, no special code required).

[deathcap]

Changed one texture (textures/blocks/vines) to grayscale as a test. MC 1.7.5 with ProgrammerArt, before (color fixed in texture):

after (gray texture, MC fills in the color):