x/tools/cmd/present: add support for sub bullets

[jerrinsg]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.11 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/jerrin/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/jerrin/gocode"
GOPROXY=""
GORACE=""
GOROOT="/home/jerrin/go-go1.11"
GOTMPDIR=""
GOTOOLDIR="/home/jerrin/go-go1.11/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build014264932=/tmp/go-build -gno-record-gcc-switches"

Go present tool does not currently support sub bullets. This proposal is to add a new syntax to support sub bullets. Lines prefixed with '-- ' will be displayed as a sub bullet.

For example, the following code block produces the slide shown below.

* Title

- bullet 1
-- sub bullet 1.1
- bullet 2
-- sub bullet 2.1
-- sub bullet 2.2

[gopherbot]

Change https://golang.org/cl/136095 mentions this issue: cmd/present: add support for sub bullets

[bcmills]

CC @adg