Added support for commandline arguments to scripts, fixes #1453

- Scripts and callbacks can be configured by adding commandline parameters:
`composer install --script-param="name1:key1=value1"`
`composer install --script-param="value2"` (key will be equal to value2)
Multiple parameters can be added:
`composer install --script-param="name1:key1=value1" --script-param="value2"`

Scripts in composer.json can have named parameters:
"scripts": {
    "post-install-cmd": [
        "myscript {name1} {value2}"
    ]
}

Events get an additional method:
- `getScriptParams()`: returns an array of commandline parameters added with --script-param
Merge branch 'script-param' of github.com:Future500/composer into script-param