Support q/Q for quarters

[borgar]

Closes #41

[borgar]

This is just sitting here because I'm not sure that I like it. 🤔

• The option name "q" is off. Probably better to use "quarters".
• The Q token is always tokenized even if the option is off.
• Is this really worth the extra complexity? Will anyone ever turn it on?
• Because q is currently a "passthrough" characters there is a big risk that a user may turn this on only to discover later that q was used normally in some pattern that was picked up from a spreadsheet. (Meaning, this is really only helpful in controlled cases where a developer is in control of the pattern.)

There is also a big question in my mind if the letter "Q" should be emitted as a part of the output or not. Quarters are pretty much never emitted without a Q prefix. So this means that anyone using quarters will always have to use three characters: \Qq.

There might be a better case for doing this by enabling the functionality not with an option but rather only using a prefix directive: [Q]yyyy\Qq. This way we can be sure that character collision never happens as any string containing the prefix will be written for the behaviour.