Update CLI to return status based on lint severity

Change behavior of the CLI to return a different status code when:

  * No errors or warnings are reported: 0
  * Only warnings are reported: 1
  * One or more errors (and any number of warnings) are reported: 2

This changes the previous non-zero error code from 65 to 1 and 2, since
there were no other "semantic" exit codes that could be used for
warnings, so we might as well use 1 and 2.

Change-Id: I5e4c262904cb5d6d89d08baa14d321da70fc0540
Reviewed-on: http://gerrit.causes.com/40806
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Add documentation for exit status codes

In order to provide better insight into what kind of exit codes
`scss-lint` can return so that other applications can rely on them,
document them explicitly in the README.

Change-Id: I317dc4dada9040d8ed14988ca19173d7bec69085
Reviewed-on: http://gerrit.causes.com/40807
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>