support $ref resolution

[ityonemo]

Add list of all known places here:

[ityonemo]

• [x] schema object
• [x] parameters object
• [x] requestBodies object
• [x] pathItems object

v 0.2+

• [ ] response object
• [ ] headers object
• [ ] securitySchemes object
• [ ] callbacks object

not needed ?:

• examples object
• links object

[ityonemo]

v1 $ref resolution completed here: 5d92d0e