Fix no project config issue

[Arcanemagus]

The changes in #41 and #42 made it so if you have no configuration (in other words are using one of the presets) you just get a message that it "Failed to parse config file" since we are requiring a config currently.

This PR changes the behavior so that if cosmiconfig is unable to find a configuration for the project the lint is still ran using the default options from the preset. If usePreset() is false it will still throw a warning message up.

[Arcanemagus]

@1000ch can you test this to make sure this still works properly with projects with a configuration?

[Arcanemagus]

Also the remaining "Uncaught (in promise) TypeError: Cannot read property 'filepath' of null", source: /home/travis/build/AtomLinter/linter-stylelint/node_modules/stylelint/dist/buildConfig.js (32) is a bug in stylelint as far as I can tell.

For some reason on import it is attempting to grab a configuration, and then when that fails it never handles that and instead assumes that it succeeded, thus causing this error when it tries to access part of the result which is null.

Note: It looks like this only happens in the v1.4.0-beta0 builds, so import behavior may have changed somehow.

[1000ch]

Tested with configuration (.stylelintrc, stylelint.config.js, stylelint section in package.json).