cmd/addchain,internal/gen: allow input parameters

[mmcloughlin]

Initial code generation support #94 #127 has landed but has some limitations, the most notable of which is that variable names are hard-coded:

addchain/cmd/addchain/gen.go

Lines 62 to 69 in 6a7d3de

	// Prepare template data.
	cfg := gen.Config{
	Allocator: pass.Allocator{
	Input: "x",
	Output: "z",
	Format: "t%d",
	},
	}

At a minimum these should be configurable.

It may also make sense to support additional input parameters that are passed on to the template. For example, in the code generation case you might want things like the function name or package name to be configurable so that one template could be shared.