Add SCSS configuration for Hound

This change uses Bootstrap's existing `scss/.scss-lint.yml` file
to configure Hound's hosted SCSS-Lint instance.

On each pull request to Bootstrap,
Hound will comment on any SCSS style violations in-line, like this:

![screenshot](https://images.thoughtbot.com/hound/scss-example.png)

If you update the pull request to adopt a suggestion,
the comment will be hidden.

It leaves the existing linting done by Grunt + Travis.

Hound is free for open source projects
and is open source itself:

https://github.com/thoughtbot/hound

.hound.yml

+java_script:
+  enabled: false
+scss:
+  config_file: scss/.scss-lint.yml
+  enabled: true
+  ignore_file: scss/_normalize.scss