Dev mode configuration, skip symlinks and extension directory options

[timofloettmann]

Relates to #21 and #38

Unfortunately I couldn't reproduce #35 on OSX.

"extra": {
    "typo3/cms": {
        "cms-package-dir": "{$vendor-dir}/typo3/cms",
        "web-dir": "web"
    },
    "typo3/cms-dev": {
        "cms-package-dir": "{$vendor-dir}/typo3/cms",
        "use-ext-dir": false,
        "skip-symlinks": true
    }
},

Both configurations can of course take all options.

• If the typo3/cms-dev section exists, it's used as long as --no-dev is not present in the argument list.
• When --no-dev is present, the "typo3/cms" section is used instead.
• If the typo3/cms-dev section does not exist, typo3/cms is always used. Regardless of --no-dev.

If "use-ext-dir" is set to false, the extensions are installed in {$vendor-dir}/typo3-ter/*

[helhum]

Thanks for the pull request!

Unfortunately it combines to many intents.

1. Adds configurable extension dir -> please remove that
2. Adds dev/no-dev distinction for configuration -> while this may be a nice addition in general, please separate that from the symlink creation feature

[timofloettmann]

Valid points. Sorry. I'll close this one and open new pull requests for the separate features soon.