Copy-edit the while-offline option and make it a boolean flag
Copy-edit the while-offline option and make it a boolean flag

This loses a tiny bit of generality, because formations like these no
longer work (which can occur in some scripts):

  $ wal-e backup-push --while-offline=true --while-offline=false ...

But I think in common cases it is easier to read, as the following
will now suffice:

  $ wal-e backup-push --while-offline

Signed-off-by: Daniel Farina <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@heroku.com>

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