Drop excessive data in test files (#11)

1.  Deleted base64-encoded image data from test / example notebooks and updated "golden" files:
  -  example/nbee/notebook.ipynb had 2 base64 images and weighted a whopping 273 KB. That's a lot of useless data to move around, considering that nbee is just an example for documentation purposes.
  - testdata/notebook.ipynb had 1 base64 image, which took up ~85% of its size -- that that image is part of the golden-file test is not essential, as rendering of images from a mime-bundle is well-tested in other unit tests
  - Finally, because of so much .ipynb kilobytes, Github would assume nb was a predominantly-Jupyter-notebook-project, which is just not right :)

2. Fixed incorrect installation guide in the README for example/nbee

example/nbee/.tool-versions

@@ -0,0 +1 @@
+golang 1.18

example/nbee/README.md

@@ -7,20 +7,30 @@
 Compile the package on the fly 🐝
 
 ```sh
-go run github.com/bevzzz/nb/example/nbee
+go run github.com/bevzzz/nb/example/nbee -f path/to/notebook.ipynb
 ```
 
 Or, install a binary 🗑
 
 ```sh
-go install github.com/bevzzz/nb/example/nbee
+# Build from source:
+git clone https://github.com/bevzzz/nb.git
+cd nb/example/nbee
+go install
+
+# Install from remote repository (Go 1.17+):
+go install github.com/bevzzz/nb/example/nbee@latest
 ```
 
 Try it out:
 
 ```sh
-nbee # convert the default notebook to HTML
-nbee -f "my_notebook.ipynb" # convert your own notebooks
+# If you already have bevzzz/nb checked out:
+cd nb/testdata
+nbee -f notebook.ipynb
+
+# Convert your own notebooks
+nbee -f my_jupyter.ipynb
 ```
 
 ## Disclaimer

example/nbee/main.go

@@ -14,29 +14,29 @@ import (
 	synth "github.com/bevzzz/nb-synth"
 	"github.com/bevzzz/nb/extension"
 	"github.com/bevzzz/nb/extension/adapter"
+	jupyter "github.com/bevzzz/nb/extension/extra/goldmark-jupyter"
 	"github.com/bevzzz/nb/render"
 	"github.com/bevzzz/nb/render/html"
-	jupyter "github.com/bevzzz/nb/extension/extra/goldmark-jupyter"
 	"github.com/robert-nix/ansihtml"
 	"github.com/yuin/goldmark"
 	highlighting "github.com/yuin/goldmark-highlighting/v2"
 )
 
 var (
-	file = flag.String("f", "notebook.ipynb", "Jupyter notebook file")
+	file = flag.String("f", "", "Jupyter notebook file")
 )
 
-//go:embed notebook.ipynb
-var defaultNotebook []byte
-
 func main() {
 	flag.Parse()
-	var err error
+	if *file == "" {
+		log.Fatal("-f must be a valid path to notebook")
+	}
 
-	b := defaultNotebook
+	var b []byte
+	var err error
 	outFile := "notebook.html"
 
-	if f := *file; file != nil {
+	if f := *file; f != "" {
 		if b, err = os.ReadFile(f); err != nil {
 			log.Fatal(err)
 		}