Respect OVERCOMMIT_DISABLE environment variable

Add support for disabling Overcommit hook runs by setting the
`OVERCOMMIT_DISABLE` environment variable to a non-zero, non-empty
value.

This is useful in scripts when you want to be able to execute some `git`
commands without Overcommit running or displaying output.

Change-Id: I494c30f92286bb714ec7d8495540e3e4837e0d66
Reviewed-on: http://gerrit.causes.com/41746
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>