Allow general backends in `BlockSkylineMatrix`

[dlfivefifty]

I have a need for non-Vector (in fact, infinite) backends in BlockSkylineMatrix so am planning to template out the storage, just as BandedBlockBandedMatrix and BlockArray allow general backends.

While I'm at it, there's a question whether we want to make the type signature more resemble the new BlockArray:

BlockArray{T, N, R <: AbstractArray{<:AbstractArray{T,N},N}, BS<:AbstractBlockSizes{N}}

That is, we have two options:

1. BlockSkylineMatrix{T, R <: AbstractVector{T}, LL<:AbstractVector{Int}, UU<:AbstractVector{Int}}
2. BlockSkylineMatrix{T, R <: AbstractVector{T}, BS<:AbstractBlockSizes{2}}

Option 2 is closer to the new BlockArray. @jagot Any reason to prefer Option 1?

[jagot]

Not that I can think of.