set the scheduler location in the cron expression and add CronWithSeconds()

[JohnRoesler]

What does this do?

• explicitly sets the location for the crontab expression using CRON_TZ - the parse expression defaults to time.Local, which doesn't handle when a caller is setting a different timezone for the scheduler
• add a new method CronWithSeconds() that the parsing of seconds which robfig exposes
  • i made this a separate method because standard cron expressions don't support seconds, so rather than have the Cron() func optionally support seconds i thought making explicit was better

Which issue(s) does this PR fix/relate to?

List any changes that modify/break current functionality

Have you included tests for your changes?

Did you document any new/modified functionality?

• Updated example_test.go
• Updated README.md

Notes