add option parameter for the generator

We cannot add option to the choosen generator.

For example with [protoc-gen-gotemplate](https://github.com/moul/protoc-gen-gotemplate)

you can add `debug` or `template_dir` option to the generator like :
`--gotemplate_out=template_dir=.,debug=true:.`

It could be cool to have an option in the `Protobuild.toml` to be able to add them something like :

```toml
[[generator]]
name = "gotemplate"

[generator.options]
debug = true
template_dir = "."
```

What do you think ?