Feature/flexible directories

This PR is related to discussion: https://github.com/farazdagi/phrozn/issues/5

In practice this PR allows an user to use Phrozn in different ways:

## Usual

```shell
phr init
phr up
phr single index.twig
phr clobber
```

In this way as usual the `.phrozn` folder was craeted at `init` and all works normally.

## Not standard folders

```shell
phr init src
mkdir htdocs
phr up src htdocs
```

The project `src` contains the Phrozn project (no `.phrozn` folder) and all the generated files are in `htdocs` folder.

### Project foldering

```
- src
  - plugins
  - entries
    - index.twig
    - ...
- htdocs
  - index.html
  - ...
```

### Clear the project folders

```shell
phr clobber src
```

This destroy the project folder for not standard structure.

# Plugins autoloading

To support this feature the plugin autoloader is loaded in order to work in the right folder.