Add lint to Gulp, break it out as a seperate task

[brendo]

This small PR exposes the existing lint tasks as a dedicated Gulp task so that we can easily check lint during development.

I've also fixed up the failing files so Travis should be happy.

[squidfunk]

I did exactly the same last night - I put the scripts into bin, because they will get longer and then it will become unreadable very quickly. Also added a lint script which uses eslint and sass-lint directly. This is faster than invoking Gulp, because Gulp first needs to transpile all tasks. IMHO it's better we leave it like that.

Furthermore, the Gulp tasks run in parallel and if one of the linting tasks fails, the other may not run. This is handled in the script bin/lint, which definitely runs both linters and then combines the exit codes.

[brendo]

Nice!