add high level interface

[ChrisRackauckas]

Adds a high level interface. See workflow from sparsity pattern:

colors = matrix_colors(sparsity_pattern)
DiffEqDiffTools.finite_difference_jacobian!(_J, f, x, color = colors)

matrix_colors is an interface that calls the underlying coloring algorithms using a dispatch form so we can just keep on adding new ones. In addition, this function can be overloaded so that specializations return an analytical solution to the colors, bypassing the graph algorithm (@huanglangwen this is what we will use to add dispatches).