User specific skeleton directory for generators

I tried to fix the issue #65 of GeneratorBundle.

How to do:

```
app/console generate:bundle --skeleton=path/to/my/skeleton
app/console doctrine:generate:crud --skeleton=path/to/my/skeleton
app/console doctrine:generate:form --skeleton=path/to/my/skeleton
```

or invoke them without `--skeleton` then specify it interactively later.