chain/SkipIfStillRunning: fix bug handling different jobs

It was an error in channel scoping that was identified in pull #263.

This adds a unit test to identify that issue and verify the fix.

docs: update and expand

parser: fix combining of Dow and Dom

Relevant portion of the spec (https://en.wikipedia.org/wiki/Cron):

    While normally the job is executed when the time/date specification fields all
    match the current time and date, there is one exception: if both "day of month"
    (field 3) and "day of week" (field 5) are restricted (not "*"), then one or both
    must match the current day.[3]

When originally written, I incorrectly allowed restricted stars like "*/10" to
trigger the "all must match" behavior. This bug would cause some schedules to
run less frequently, or not at all.

Fixes #70

Update doc.go to fix description of @every

Update doc.go to fix description of @every

Fix usage example

Fixes #11