+ In the `unit_kw` grammar object, test for a prior match
  properly when looking for prefixes by n-gram (it was
  testing whether a unit had been detected).
+ When a match is found in `System::evaluateUnitPrefixToken()`,
  stop iterating so the iterator is left in a valid state.
+ Prefixes should never have a plural form.
+ Add a `Unit::multiplier()` method for regression testing purposes.
+ Add regression tests.
+ Add a feature to the `unit-converter` utility program: when
  passed a measurement and unit on the command line, perform
  the conversion as specified and exit rather than processing
  user input.