fix(index.js): -x flag is not working properly for the suppression case

When passing `-x -` on the command line, a schema is still created in the process' cwd.
This is because the `-x` value of `-` is coerced to an empty string.
Later on, it is tested against '-' instead of empty string.

Fixes #203

lib/index.js

@@ -66,7 +66,7 @@ async function main(args) {
  .alias('x', 'schema-out')
  .describe('x', 'output JSON Schema files including description and validated examples in the specified folder, or suppress with -')
  .default('x', nodepath.resolve(nodepath.join('.', 'out')))
- .coerce('x', x => (x === '-' ? '' : nodepath.resolve(x)))
+ .coerce('x', x => (x === '-' ? '-' : nodepath.resolve(x)))
 
  .alias('e', 'schema-extension')
  .describe('e', 'JSON Schema file extension eg. schema.json or json')