Add --format flag to Zally CLI

In order to make Zally Command Line Utility (CLI) usable in different Continuous Integration platforms, we want to add more formatting options. Therefore, we need to implement an extra --format flag.

Acceptance Criteria

As a Zally CLI user
I want to provide the --format flag when running CLI
So that Zally will format its output in plain text or markdown

Example

./zally --format=markdown

Expected text output:

MUST Use Problem JSON
  Operations Should Return Problem JSON When Any Problem Occurs During Processing Whether Caused by Client Or Server
  https://zalando.github.io/restful-api-guidelines/#176
    /products GET 429
    /products/{product_id} GET 418
    /products/{product_id} GET 429
    /products/{product_id} PATCH 400
    /products/{product_id} PATCH 429

Expected markdown output

### `MUST` [Use Problem JSON](https://zalando.github.io/restful-api-guidelines/#176)

> Operations Should Return Problem JSON When Any Problem Occurs During Processing Whether Caused by Client Or Server
        
- /products GET 429
- /products/{product_id} GET 418
- /products/{product_id} GET 429
- /products/{product_id} PATCH 400
- /products/{product_id} PATCH 429

Expected output when no violations happened:

Congratulations! No violations found.

[netme]

Thanks @maxim-tschumak and @roxspring for the valuable input. Let me proceed with the implementation then

[maxim-tschumak]

Looks good!

Should we also distinguish between text output with colors and without colors?

Let’s keep it simple 😃

Should “no violation” differ in markdown / text outputs?

IMHO, it can be the same

How about Short / Long / Custom formats for every single violation (example: git log --pretty)

Great idea!