Add format option for whitespace after/before symbol (#248)

I've added a new format option called `:symbol_before_without_space`. The default value is `true` for backward compatibility. If set to `false`, a whitespace is inserted between the symbol and the amount (if `:symbol_position` is `:before`).

Also, I noticed that there is an existing option to control the behaviour when `:symbol_position` is `:after`. This option is called `:symbol_after_without_space` and its default value is `true`.

I've added tests for both the new option and the existing one. Also, I've added them to the yard docs of the `format` method.

Related to #248.