Support template arguments

[LunarWatcher]

This may be required for languages like Python, where we can't or don't want to infer all the information from the current repository.

Essentially, by doing :DawnGenerate someTemplate name=someArgument, someTemplate gets access to %name. I'm not entirely sold in this approach specifically, though, because it feels clunky and out of place in a command. Another implementation idea is using popups, but that's gonna require vim/nvim compatibility, because nvim has a different API.

Other implementation ideas are also welcome - especially wrt. templates "requesting" arguments to be present.