image: refactor; add more color variations.

Refactor store into its own file.

Accept the length of the random sequence of numbers.

Call Collect on a separate goroutine.

Update documentation.

cmd: use standard width and height for image.

Document functions and types in captcha.go.

Refactored image captcha ready, and documented.

Numbers are now laid out correctly.

Next commits will introduce perturbations.

Started work on code cleanup.

Gofmt font.go.

Move image-related functions into image.go.

Add line that strikes through digits.

Reorganize code. Add storage.

Initial version (numbers).