Add support for XML command-line switch

Adds support for outputting XML for lint error messages via the '-x' or
'--xml' command-line switches.

Closes #1

Change-Id: I28be6c42a2285d5626ced07f7809e1ef181bf340
Reviewed-on: https://gerrit.causes.com/19667
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Add support for flag to exclude files

Adds support for excluding files with the '-e' and '--exclude' switches.

Closes #2

Change-Id: I1a7776a8e73a4617cad6b3d8ef13c4a7cd5aa150
Reviewed-on: https://gerrit.causes.com/19693
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Add support for ignoring linters

Adds support for ignoring linters via the '-i' or '--ignored-linters'
command-line switches.

This widens the potential audience of people who use `scss-lint` as they
can specify which kinds lints they want to get warnings about.

Closes #3

Change-Id: I278b6179236f985aaa47a8ce3e3555b7a56b1628
Reviewed-on: https://gerrit.causes.com/19694
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>