Allow the binary finder to use the project's composer.json file

- Load JSON file and get bin-dir value: https://getcomposer.org/doc/articles/vendor-binaries.md

- Update finder behaviour to avoid code duplication

Signed-off-by: Stéphane HULARD <s.hulard@chstudio.fr>
Merge pull request #475 from CHStudio/composer-config-bindir

Allow the binary finder to use the project composer.json