Add support for nested configuration

[DannyBen]

cc #74

This PR adds support for nested configuration:

git:
- --help
- --version
- init:
  - --help
  - --bare
- checkout:
  - --help
  # + prefix is a wildcard (can also use "*--branches" instead, but in quotes)
  - +--branch: &branches
    - dev
    - master
  - +-b: *branches

This configuration will be internally converted to the "flat" config:

git:
- "--help"
- "--version"
- init
- checkout
git init:
- "--help"
- "--bare"
git checkout:
- "--help"
- "--branch"
- "-b"
git checkout*--branch:
- dev
- master
git checkout*-b:
- dev
- master

Notes:

• Asterisks for flag completion need to be specified at the beginning of any flag that requires it: - "*--branch"
• Since asterisks have special meaning in YAML, you can also use + instead: - +--branches - this way, there is no need for quotes.
• Items that start with a wildcard (* or +) will be stitched to its prefix without a leading space.

This change should be backwards compatible - so anyone can use either the flat or nested config, or even mix and match the two in the same file.