Custom Environment Path

[tobias-dev]

I'm wondering if would be good to have a option in the humbug config file to be able to add a specific system environment path, e.g.:

"environment": {
    "path": [
        "vendor\/bin"
    ]
}

So this path would be added to the environment path variable, so that phpunit e.g. would be found there. I think it might be quite useful especially in the case that humbug is installed with composer.

[padraic]

For reference, see #253 which allows a custom phpunit file - not actually a searchable path to be prefixed to PATH, or to be the sole searchable path allowable.