Added "-dev" for extra values

[JeroenBoersma]

After recent events it should be awesome to have different strategies for dev and production.
When working locally you can now have any extra parameter, append a -dev and these will overwrite the other setting.

For example:

....
"extra": {
    "magento-deploystrategy": "copy",
    "magento-deploystrategy-dev": "symlink",
}
...

[JeroenBoersma]

Fixes #162

[Flyingmana]

thank you very much, thats a nice solution :)

A followup PR adding some documentation in the Docs (maybe the FAQ section) and maybe some tests would be appreciated.

[JeroenBoersma]

If there is something I can do, just hit me up... Happy to help!