Disallow inverted ranges like 20-4

[atifaziz]

Disallow inverted ranges, as in 20-4 * * * *. It confuses folks (see #24 and #14) and most other implementations don't allow it either.

This would be a breaking change so it should be done in the next major version.

[atifaziz]

In #31, @skewwhiffy said:

Happy to try and fix this, just let me know.

Thanks for the offer to try & fix. Just keep in mind that this issue proposes a breaking change so it's only going to be rolled into the next major release and I don't have a schedule for that yet.

[skewwhiffy]

@atifaziz Is it worth fixing the FRI-MON issues separately, as it's not immediately obvious that this is an inverted range issue?

[atifaziz]

@skewwhiffy The code to parse ranges is the same for all fields of the crontab expression so if you fix it for one, it's fixed for all.

[skewwhiffy]

@atifaziz I know, I'm proposing separating them because I don't think that 'FRI-MON' should be treated in the same way as '5-1': the latter looks and feels like an inverted range, but the former, at least to humans, does not.