config: add yaml flag

[AlexanderYastrebov]

YAML flag parses (preferably flow-style) YAML value from the command line or unmarshals object yaml value from the yaml config file.

Example value:

bin/skipper -foo-flag='{foo: hello, bar: [world, "1"], baz: 2, qux: {baz: 3}}'

and equivalent branch in config yaml:

foo-flag:
  foo: hello
  bar:
    - world
    - "1"
  baz: 2
  qux:
    baz: 3

This will be useful for #2104

It is also a better alternative to manual parsing of micro-syntax like e.g. implemented in #2888

[szuecs]

:+1:

[AlexanderYastrebov]

:+1: