`preferunits` without global state

Currently, `preferunits` works by changing the values in the global dictionary `Unitful.promotion`. This causes problems with precompilation (#545, #600).

My suggestion for Unitful v2.0 is to replace the global "unit preference dictionary" (`Unitful.promotion`) by an object that the user creates and explicitly uses to convert to their preferred units. Instead of
```julia
preferunits(cm)
upreferred(1m)
```
the user would do the following:
```julia
my_unit_preference = UnitPreference(cm) # this returns a UnitPreference object, does not alter global state
my_unit_preference(1m) # the UnitPreference object is callable
```

However, this means that automatic promotion would not promote to the user’s preferred units but always to the default units (unless they are using `ContextUnits`/`FixedUnits`).

Introducing this new `UnitPreference` object can be done in a minor release, but removing the possibility of setting a global unit preference would be breaking.