Feat additionnal project yaml

[JuPrgn]

Fix #117

Add a feature to use Ceedling project: argument in order to allow to merge/load custom configuration from an additional project_specific.yml to the main project.yml.

This is usefull to test on different compiler or for different hardware as you can keep you common settings on main project.yml and use multiple specific.yml files to changes flags, defines... at runtime.

[numaru]

The data in project.yml is also read by the extension and any additional .yml file merged at runtime should be considered the same.

• The translation from the option names to .yml files should be done using the [:project][:options_paths] setting in project.yml
• The option files should be read and merged in getYmlProjectData()
• The option files should be watched for change by watchFilesForReload()

[numaru]

Thank you for the contribution. I prefer to not include the PR in today's release because I think we can improve the support for options files and I don't want to change the ceedlingExplorer settings too often.

[JuPrgn]

Thank you for your feedback I will improve this PR soon following your recommendations. If I understand it well you propose to merge optional data to a single project.yml that will be used by getYmlProjectData() ? Merging project.yml and optional.yml datas could be an issue if some key are defined twice ? this is a check that is performed by the extension right ?