allow custom fennel version

[jaawerth]

It's important to allow a user to specify an instance of fennel to use, but there are a few approaches that only differ by available and default behaviors.

Should configurable fennel version/instance:

1. Default to using the first instance found on the lua package path, with config options included for the user to have it use the included fennel instance or a path to have it use the specified version?
2. Default to the included fennel instance with an override option for a custom version and a "use package.path" option?
3. Not include a fennel instance at all, and require it to either be specified or already be on package.path?

I am leaning towards option 1 or 2, but would welcome feedback. Thanks!

[bakpakin]

Seems to me it would probably be easiest to default to whatever is found on the package path, but use dofile('user-provided-path-to-fennel') for a user provided path. A user could mess with package.path in the nvim/init.vim file if they needed to mess with the package paths, I think.

[jaawerth]

That makes sense; no need to add extra config that just wraps package.path manipulation when I could just document the necessary change to package.path. And it could fallback to the include fennel version just by sticking its location to the end of package.path.

Thanks!

[meh]

What's the trick to use a more recent version of Fennel?