Allow loading preludes or extending previous unit_system calls

[Tehforsch]

It would be nice to be able to do something like

unit_system!(
    load "si_prelude",
    unit foo = 12 * meters,
)

in order to extend pre-defined systems of units with a few custom units.

Another option could be to allow calling unit_system (or rather a stripped down version of the macro) again in order to add units to the previously generated Quantity type, such as

unit_system!(
    ...
)

add_units!(
    unit foo = 12 * meters,
)

However this requires persistence across multiple macro calls which is non-trivial (see rust-lang/rust#44034)