Consider making handlebars optional

[tmpfs]

Hi @miquels,

I just tried this out in my project using warp-compat and it's working great, thanks 👍

I took a quick look around the code and was wondering whether it would be possible to remove the hard dependency on handlebars. I would be happy to do the legwork to add a handlebars feature flag if you would take a PR.

We could make the handlebars feature part of the default features so the change would be backwards compatible.

It appears the parsing of .hbs files as handlebars is not critical to the primary function of the library but of course I may misunderstand things.

Cheers 👍

[Jikstra]

I'm a bit suprised that there's a handlebars template engine at all in this crate. I guess there's some good use cases for it, but doubt that it should be turned on by default and probably integrated on top of using this crate?