Adding getDefaultConfig function to core

[raphaelokon]

Addresses #749

Summary of changes:
Adding a (static) function to the patternlab core to allow users to obtain a default config object

[coveralls]

Coverage increased (+0.04%) to 72.563% when pulling d892c4e on feature/749-default-config into c964462 on dev.

[bmuenzenmeyer]

tested using

const patternlabModule = require('@pattern-lab/patternlab-node');
const config = patternlabModule.getDefaultConfig();
const patternlab = require('@pattern-lab/patternlab-node')(config);

...

perfect 👍