Support natural language offsets

[jeromedalbert]

At work we have a cron job that runs every 15 minutes (minutes 0, 15, 30, 45). The corresponding natural language expression is easy to write: every 15 minutes.
But we also have a second job that runs 1 minute after the first job (minutes 1, 16, 31, 46). There is currently no nice way to express this in natural language besides listing all the minutes: at minutes 1,16,31,46.

I thought it might be nice if the natural language parser could support offsets, so the second job's schedule could be rewritten to every 15 minutes starting at minute 1 or every 15 minutes from minute 1.

This PR is an attempt to support such offsets. If this looks useful, I am happy to tweak this further. For example I sort of like the expressiveness of "starting at", but it might look too wordy.

[jmettraux]

Salut, thanks for the PR, looking good, let me sleep and marinate over it...

[jeromedalbert]

Merci pour ta considération et pour cette gem très utile que j'utilises depuis des années !

Some extra info:

• the word "minute" is repeated in every 15 minutes starting at minute 1 because it looks more natural to me than every 15 minutes starting at 1, but I can add support for omitting the second "minute" word. I can also add support for a sentence like every 15 minutes with offset 1.
• I am not currently validating that the second "minute" word matches the first one. In the incorrect sentence every 15 minutes starting at hour 1, the offset is still interpreted as a minute offset. This may feel like an edge case so I kept the PR simple, but I can add some validation if desired.
• The PR's initial focus was offsets, but looking at your exploration in 21cb1c9 we could support ranges like every 15 minutes from minute 1 to 31. In that case what I call offset is just a subset of the notion of ranges, which is the official term according to man 5 crontab. So we could refactor the PR with names like range instead of offset, with an optional range end. This is probably the cleaner way to go.

[jmettraux]

Salut,

I'm taking it in, I might tweak it a little bit, but overall I like your work.

Merci beaucoup !