Add shell filter by active branch

[valcriss]

Contribution Type: new feature

This pull request affects the following areas:

• [ ] Front-End
• [ ] Builder
• [X] Build Plugins

In raising this pull request, I confirm the following (please check boxes):

• [X] I have read and understood the contributing guidelines?
• [X] I have checked that another pull request for this purpose does not exist.
• [X] I have considered, and confirmed that this submission will be valuable to others.
• [ ] I have created or updated the relevant documentation for this change on the PHPCI Wiki.
• [X] Do the PHPCI tests pass?

Detailed description of change:

Hello, I had to limit the execution of the shell plugin on a specific branch, I modified slightly the structure of the parameters of the plugin to have a series of commands per branch.

I do not know if you will find this feature interesting.

[dancryer]

This can already be achieved as follows:

branch-master:
    run-option: 'after'
    test:
        shell:
            - "echo 1"