Bundle split config files into a single YAML document

## Usage

`circleci config pack <path> [flags]`

## Arguments

`<path>` is the path to a split config directory to pack,
for example, `.circleci` or `src/ci`. The directory structure is
mapped to YAML keys in the merged document.

## Flags

Global flags: `-c, --config`, `--debug`, `--no-color`, `-q, --quiet` — see `circleci --help`.

## Examples

- Pack the default config directory: 
  `circleci config pack .circleci`
- Pack and pipe to validate: 
  `circleci config pack .circleci | circleci config validate --config -`
- Pack a custom directory: 
  `circleci config pack src/ci`

## Details

Merge a split CircleCI config directory into the single YAML document CircleCI
accepts, printed to stdout.

The directory structure maps to YAML keys: .circleci/config.yml merges at the
top level, and .circleci/jobs/build.yml becomes jobs.build. Files whose names
begin with "@" merge at the current level instead of nesting under a key.

