$> go get github.com/vbatts/goapi

$> goapi -h
Usage of goapi:
  -allow_new
        allow API additions (default true)
  -c string
        optional comma-separated filename(s) to check API against
  -contexts string
        optional comma-separated list of <goos>-<goarch>[-cgo] to override default contexts.
  -except string
        optional filename of packages that are allowed to change without triggering a failure in the tool
  -gopath
        include package names from GOPATH as well
  -next string
        optional filename of tentative upcoming API features for the next release. This file can be lazily maintained. It only affects the delta warnings from the -c file printed on success.
  -v    verbose debugging

$> goapi -gopath github.com/vbatts/tar-split/tar/asm > asm-api.txt
$> cat asm-api.txt
pkg github.com/vbatts/tar-split/tar/asm, const OptFileCheck = 1
pkg github.com/vbatts/tar-split/tar/asm, const OptFileCheck Options
pkg github.com/vbatts/tar-split/tar/asm, const OptSegment = 2
pkg github.com/vbatts/tar-split/tar/asm, const OptSegment Options
pkg github.com/vbatts/tar-split/tar/asm, const OptVerify = 4
pkg github.com/vbatts/tar-split/tar/asm, const OptVerify Options
pkg github.com/vbatts/tar-split/tar/asm, func NewInputTarStream(io.Reader, storage.Packer, storage.FilePutter) (io.Reader, error)
pkg github.com/vbatts/tar-split/tar/asm, func NewOutputTarStream(storage.FileGetter, storage.Unpacker) io.ReadCloser
pkg github.com/vbatts/tar-split/tar/asm, type Options int
pkg github.com/vbatts/tar-split/tar/asm, var DefaultInputOptions Options
pkg github.com/vbatts/tar-split/tar/asm, var DefaultOutputOptions Options
$> goapi -gopath -c asm-api.txt github.com/vbatts/tar-split/tar/asm
$> echo $?
0

$> goapi $(go list crypto/...)
pkg crypto, const MD4 = 1
pkg crypto, const MD4 Hash
pkg crypto, const MD5 = 2
pkg crypto, const MD5 Hash
[...]
pkg crypto/aes, const BlockSize = 16
pkg crypto/aes, const BlockSize ideal-int
pkg crypto/aes, func NewCipher([]uint8) (cipher.Block, error)
pkg crypto/aes, method (KeySizeError) Error() string
pkg crypto/aes, type KeySizeError int
pkg crypto/cipher, func NewCBCDecrypter(Block, []uint8) BlockMode
pkg crypto/cipher, func NewCBCEncrypter(Block, []uint8) BlockMode
[...]