Add build subcommand, from stdin flag for apply

Hey guys!

You can now use flavours as a traditional base16 builder: the subcommand flavours build builds a given template file using a scheme file. Perfect for any cool automation you can think of.

The flavours apply subcommand now supports a --stdin flag. This can be used in all sorts of ways, but the reasoning for this addition was chaining with flavours generate, allowing you to generate and apply a scheme without writing it to disk first (works great if you have an immutable flavours directory, for example): flavours generate --stdout dark kitties-wallpaper.png | flavours apply --stdin.

I've also revamped how shell completion generation works, no more manual fixing nescessary!