Add support for relative '../' paths and ignore commented !includes

[Jbelmonte]

Hi there!

First of all, thank you for your merge solution! I needed that to import RAML into Postman.
Secondly I must say I am not a PHP developer, so please forgive any bad use of PHP.

This pull request contains the following changes/improvements:

• Add support to local file paths referencing parent folders.
  • I have a complex RAML specification where I refer to yaml in nested folders and them refer to parent folders.
    Bellow you'll find an example.
• Refactor code so it handles files line by line:
  • It eases ignoring commented (#) !include lines. Includes were processed anyway, causing syntax errors as the first line remained commented.
  • Handle !include in all available situations: traits, securitySchemes, resourceTypes and so on.
• Remove extra \n on each line.

File structure

~/api/api.raml
  |- /resources
  |      |- ...
  |- /yaml
  |      |- /traits.yaml
  |      |- ...
  |- /yaml-core
  |      |- /traits/trait.localizable.yaml
  |      |- ...

Cross references

# Fragment of `api.raml`
traits: !include yaml/traits.yaml

# Fragment of `/yaml/traits.yaml`
- !include ../yaml-core/traits/trait.localizable.yaml

# Fragment of `/yaml-core/traits/trait.localizable.yaml`
localizable:
    headers:
      Accept-Language:
        description: Language code.
        example: es

The problem

$  php ramlMerge.php api.raml
[...]
# Unable to Include ~/api/../yaml-core/traits/trait.localizable.yaml
[...]

The folder containing referenced file ../yaml-core/traits/trait.localizable.yaml had to be computed based on current file being included (/yaml/traits.yaml).

Of course, that complex specification was valid according to RAML parsers used by RAML designer and RAML console.