Add composer_path as a parameter to composer commands

By default `composer/install` executes composer path as `composer.phar` in project path. It's a bit problematic, when composer is installed globally or in custom path. 
This PR includes customizable `composer_path`, so calling `composer/install` task is as simple as:

```
tasks:
  pre-deploy:
    - composer/install: {composer_path: 'composer'}
```

Tested and working. Feel free to review!