Added single-page command

Hi Victor,

I've added a feature that's allows to regenerate a single page.

```shell
phr single path/to/single.twig [SRC] [DEST]
```

In this way just one file is updated.

I want, if is possible, to discuss with you and any other that use Phrozn, on this feature.

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This pull request is only a simple starting point to open a discussion, don't merge.