Complete the parser

[edigaryev]

#58 introduces a YAML parser that's able to run most of the CLI tasks without network connection.

However, this results in at least two parsers in-the-wild (the other being the one that processes .cirrus.yml in the cloud). To avoid the divergence in the future, it might be a good idea to consolidate everything as a CLI package, which is available for use in any Go program.

To do this, the following things need to be implemented:

• support all execution environments
• support JSON Schema
• integrate already existing tests

[fkorotkov]

Almost everything done except JSON schema. Created a separate issue #148