Include lint severity in output

Modifies both the `DefaultReporter` and `XMLReporter` to output the
actual severity of the lint reported.

In the process, the colors output by the `DefaultReporter` have been
modified to allow the severity to take on the yellow and red colors to
represent a warning and an error, respectively. Most consumers will not
notice a difference since `scss-lint` doesn't output color escape
sequences unless output is sent to a tty.

Change-Id: I7e00efac604ea65ffcbbc10278def728167dc698
Reviewed-on: https://gerrit.causes.com/29750
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>