Make Template dir configurable, for easy templates overrides (UI engines extensions writing)

### Background

I'm writing a Bulma support extension for hanami-cli. To make it possible, I need to duplicate some commands, generators and templates, most of which are ruby files that I can safely ignore. This happens, because all our generators use `__dir__`, to fetch the source file to generate content from.

```ruby
def template(path, context)
require "erb"

ERB.new(
File.read(File.join(__dir__, "app", path)),
trim_mode: "-"
).result(context.ctx)
end
```

### TODO

Make the `__dir__` configurable, so gem authors can easly change the `base_path` for generators. If file does not exists, fall-back to default Hanami::CLI path.

### Pros

- Writing frontent engine support or file interpreters spport will be very easy (bulma, boostrap, or slim/haml, etc)
- We can minimize the number of files needed to be copied and kept in sync.