document what it means for a move to be valid and legal

factor check for move validity into Board.Apply

improve formatting in Board.String

add LICENSE file

cmd/tictactoe: move main to main.go

player/perfect: use image of fancy gopher

.travis.yml: use go vet instead of go tool vet

add go.mod and go.sum files

cmd/tictactoe: split terminal/browser code, add WebAssembly support

player/perfect: Add package.

Various minor improvements.

cmd/tictactoe: Use document.ReadyState to not miss DOMContentLoaded event.

cmd/tictactoe: Export page struct fields.

cmd/tictactoe: Make board clickable only when needed.

cmd/tictactoe: Display player whose turn it is in bold.