Add ability to specify unique prefixes in parsing (Code freeze 10:00am Friday)

[tze088]

Fixes #226

When parsing using ArgmentTokenizer.tokenize(), a Prefix can be designated as unique by calling .once(). For example, tokenize(argString, PREFIX_DESCRIPTION.once()).
This means that there should only be one ocurrence of this prefix in the input, meaning that:

• Adjacent duplicate prefixes are ignored, and the first occurrence is taken as the start of the argument. For example, d/description d/ will parse as description d/
• If there are two duplicate prefixes split in the middle by another specified prefix, then duplicate arguments will be generated. For example, d/1 e/1 d/2 will create 2 arguments for d/.
  • Therefore, duplicate prefixes should still be checked for.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!