Adds a Composer script to create config dir after install

I got the following PHP Warning when trying to run `./bin/neo db:list` (because the `~/.neotoolkit` dir does not exist):

```
jeremykendall at orgrimmar in ~/dev/neo4j-toolkit on master
$ ./bin/neo db:list
-----------------------
NeoToolkit Version 0.1.0-dev
This is a development version, do not use in production environment
-----------------------
PHP Warning:  file_put_contents(/Users/jeremykendall/.neotoolkit/config): failed to open stream: No such file or directory in /Users/jeremykendall/dev/neo4j-toolkit/src/Factory.php on line 60
PHP Stack trace:
PHP   1. {main}() /Users/jeremykendall/dev/neo4j-toolkit/bin/neo:0
PHP   2. Symfony\Component\Console\Application->run() /Users/jeremykendall/dev/neo4j-toolkit/bin/neo:22
PHP   3. Symfony\Component\Console\Application->doRun() /Users/jeremykendall/dev/neo4j-toolkit/vendor/symfony/console/Symfony/Component/Console/Application.php:126
PHP   4. Symfony\Component\Console\Application->doRunCommand() /Users/jeremykendall/dev/neo4j-toolkit/vendor/symfony/console/Symfony/Component/Console/Application.php:195
PHP   5. Symfony\Component\Console\Command\Command->run() /Users/jeremykendall/dev/neo4j-toolkit/vendor/symfony/console/Symfony/Component/Console/Application.php:874
PHP   6. Neoxygen\NeoToolkit\Terminal\ListCommand->execute() /Users/jeremykendall/dev/neo4j-toolkit/vendor/symfony/console/Symfony/Component/Console/Command/Command.php:252
PHP   7. Neoxygen\NeoToolkit\Factory::getConfig() /Users/jeremykendall/dev/neo4j-toolkit/src/Terminal/ListCommand.php:44
PHP   8. Neoxygen\NeoToolkit\Factory::createConfigFile() /Users/jeremykendall/dev/neo4j-toolkit/src/Factory.php:47
PHP   9. Neoxygen\NeoToolkit\Factory::setConfig() /Users/jeremykendall/dev/neo4j-toolkit/src/Factory.php:70
PHP  10. file_put_contents() /Users/jeremykendall/dev/neo4j-toolkit/src/Factory.php:60

Warning: file_put_contents(/Users/jeremykendall/.neotoolkit/config): failed to open stream: No such file or directory in /Users/jeremykendall/dev/neo4j-toolkit/src/Factory.php on line 60

Call Stack:
    0.0002     239968   1. {main}() /Users/jeremykendall/dev/neo4j-toolkit/bin/neo:0
    0.0094    1695064   2. Symfony\Component\Console\Application->run() /Users/jeremykendall/dev/neo4j-toolkit/bin/neo:22
    0.0117    2031216   3. Symfony\Component\Console\Application->doRun() /Users/jeremykendall/dev/neo4j-toolkit/vendor/symfony/console/Symfony/Component/Console/Application.php:126
    0.0118    2032136   4. Symfony\Component\Console\Application->doRunCommand() /Users/jeremykendall/dev/neo4j-toolkit/vendor/symfony/console/Symfony/Component/Console/Application.php:195
    0.0118    2032608   5. Symfony\Component\Console\Command\Command->run() /Users/jeremykendall/dev/neo4j-toolkit/vendor/symfony/console/Symfony/Component/Console/Application.php:874
    0.0120    2036272   6. Neoxygen\NeoToolkit\Terminal\ListCommand->execute() /Users/jeremykendall/dev/neo4j-toolkit/vendor/symfony/console/Symfony/Component/Console/Command/Command.php:252
    0.0125    2053888   7. Neoxygen\NeoToolkit\Factory::getConfig() /Users/jeremykendall/dev/neo4j-toolkit/src/Terminal/ListCommand.php:44
    0.0125    2054112   8. Neoxygen\NeoToolkit\Factory::createConfigFile() /Users/jeremykendall/dev/neo4j-toolkit/src/Factory.php:47
    0.0125    2054608   9. Neoxygen\NeoToolkit\Factory::setConfig() /Users/jeremykendall/dev/neo4j-toolkit/src/Factory.php:70
    0.0136    2239528  10. file_put_contents() /Users/jeremykendall/dev/neo4j-toolkit/src/Factory.php:60

No Neo4j instances registered
```

This PR adds a [Composer script](https://getcomposer.org/doc/articles/scripts.md) that runs during the `post-install-cmd` event to create the missing config dir so that `file_put_contents` won't blow up.