Validation of extensions

[ransome1]

Hi @jmhobbs .

Hope you're doing well. Happy Christmas to you and you family.

Here comes a last request for this year :) It would be good to have a way of validating if the value of an extension matches a rule set.

A good example would be recurrences, which are used in many todo.txt apps. A recurrence in sleek can look as simply as rec:d, but can also be rec:+2m. However it cannot be rec:1z, since recurrences in sleek are limited to d, b, w, m and y.

If one could define a regex for a specific extension, we could avoid jsTodoTxt detecting values, which don't make sense for specific use cases.