Passing config to CLI causes assertion error

[DanPurdy]

When using the CLI and --config an assertion error is generated if you call sass-lint from a directory that doesn't include a .sass-lint.yml file or a package.json with a config file specified within.

dan@Dans-Box workspace $ sass-lint --config ~/workspace/sass-lint/.sass-lint.yml ~/vagrant/www/vhosts/test-proj.dev/public/app/scss/main.scss

assert.js:86
  throw new assert.AssertionError({
        ^
AssertionError: missing path
    at Module.require (module.js:363:3)
    at require (module.js:384:17)
    at module.exports (/usr/local/lib/node_modules/sass-lint/lib/config.js:45:12)
    at Function.sassLint.getConfig (/usr/local/lib/node_modules/sass-lint/index.js:18:10)
    at Function.sassLint.format (/usr/local/lib/node_modules/sass-lint/index.js:89:21)
    at detectPattern (/usr/local/lib/node_modules/sass-lint/bin/sass-lint.js:17:20)
    at /usr/local/lib/node_modules/sass-lint/bin/sass-lint.js:58:5
    at Array.forEach (native)
    at Object.<anonymous> (/usr/local/lib/node_modules/sass-lint/bin/sass-lint.js:57:16)
    at Module._compile (module.js:460:26)

Have a working fix ready to go, will just have a look at adding some tests