Embed data files in the binary (#180)

Now that we're on Go 1.16+ we can do this easily! The main advantage is
it means users can build a genqlient binary and use that portably (or we
could distribute one, or whatever). Plus the code is marginally simpler;
the `embed` API is really quite nice.

Fixes #9.

Test plan:
```
make check
go build .
rm -rf generate               # pretend we have no checkout
./genqlient ./internal/integration/genqlient.yaml
./genqlient --init            # fails after generating a default config
```

generate/config.go

@@ -1,8 +1,8 @@
 package generate
 
 import (
+	_ "embed"
 	"go/token"
-	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -123,22 +123,11 @@ func ReadAndValidateConfigFromDefaultLocations() (*Config, error) {
 	return ReadAndValidateConfig(cfgFile)
 }
 
+//go:embed default_genqlient.yaml
+var defaultConfig []byte
+
 func initConfig(filename string) error {
-	// TODO(benkraft): Embed this config file into the binary, see
-	// https://github.com/Khan/genqlient/issues/9.
-	r, err := os.Open(filepath.Join(thisDir, "default_genqlient.yaml"))
-	if err != nil {
-		return err
-	}
-	w, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o644)
-	if err != nil {
-		return errorf(nil, "unable to write default genqlient.yaml: %v", err)
-	}
-	_, err = io.Copy(w, r)
-	if err != nil {
-		return errorf(nil, "unable to write default genqlient.yaml: %v", err)
-	}
-	return nil
+	return os.WriteFile(filename, defaultConfig, 0o644)
 }
 
 // findCfg searches for the config file in this directory and all parents up the tree