Yarn support

[flisboac]

First, I'd like to thank you guys for providing such an useful tool!

Is there any support for Yarn as a package manager, for the generated projects?

I see that there's a npm-install step. I just wonder if template-kit can, for example, force usage of yarn, or detect whether the template has a yarn.lock and use yarn instead of npm.

I understand that most of the time it doesn't matter which tool installs the dependencies, because in the end, all of them will end up in node_modules anyway. But it'd be cool to at least support lock files from other package managers. Also, some package managers may deal with dependency installations a bit different from the others (e.g. symlinks)

If there's no support for Yarn, or other package managers, is there at least some workaround?