Algernon 1.15.5

• Compile with Go 1.21.5 which has "security fixes to the go command, and the net/http and path/filepath packages, as well as bug fixes to the compiler, the go command, the runtime, and the crypto/rand, net, os, and syscall packages." See the Go issue tracker for more details.
• Also display images when viewing a local Markdown document with algernon -m.
• Fix a typo in the moonscript sample (thanks @testwill).
• Minor improvements to the Makefile and to the form/registration example.
• Update documentation, ref #141, (thanks for reporting @ptman).
• Update dependencies.
• Fix a test.