Add version info and URL to nix output

Thanks @grahamc for the idea.

http://lists.science.uu.nl/pipermail/nix-dev/2016-July/020958.html

src/bower2nix.ts

@@ -293,6 +293,7 @@ async function parseBowerJson(filename: string): Promise<DepResult[]> {
 
 function writeHeader(output: NodeJS.WritableStream) {
   // please note: program output is all yours, NOT copyrighted or covered by GPL
+  output.write(`# Generated by bower2nix v${version} (https://github.com/rvl/bower2nix)\n`);
   output.write("{ fetchbower, buildEnv }:\n");
   output.write("buildEnv { name = \"bower-env\"; ignoreCollisions = true; paths = [\n");
 }