More modular structure

There are many units defined by `Unitful` itself, and as I see it gives no way to disable them. Some of the default units have very short generic names, even though these units themselves are mostly used in relatively specific areas. Some examples are `u"G"` (gravity constant), `u"R"` (molar gas constant), `u"P"` (poise), `u"B"` (bel) and so on. Personally I often wish I could make `u"G"` to mean Gauss instead.

I think it would be more convenient to have something like "UnitfulBase.jl" as a separate package that sets up the infrastructure but defines no units. Then "Unitful" itself would use that package and basically consist of a single file `pkgdefaults.jl`, and other packages would have complete freedom to define units that suit any possible domain.
Does this seem reasonable to you?