Refactor `NumberToPercentageConverter`

Break down the #convert method into smaller components for clarity.
Refactor `NumberToDelimitedConverter`

Clarify the pieces of `NumberToDelimitedConverter` with better
names and with more idiomatic Ruby.
Refactor unnecessary duplication of Hash#symbolize_keys

All of these converters expect the options to be symbolized,
so always symbolize them right from the start.