Make path to endpoints.yaml configurable

[php-coder]

We shouldn't assume that endpoints.yaml always reside on the same level as the generated sources. It should be possible to generate sources in another directory.

Ways to consider:

1. always put generated sources into src/
2. allow to specify a path to endpoints.yaml so the users can do something like this: cd src; query2app ../endpoints.yaml and the source will be generated within a current directory
3. introduce an option for the directory where to place source code: query2app --dest-dir src or query2app --output-dir src

[php-coder]

--dest-dir option has been introduced.