Add convenient way to create YAML nodes in code

[kpodsiad]

Aims to close #68
Will be closed in a follow-up PR.

Current state: one can write sth like this

MappingNode(
      "hr"   -> ScalarNode("65"),
      "avg"  -> ScalarNode("0.278"),
      "blob" -> ScalarNode("nostradamus")
    )

I was thinking about implicit conversions that would allow more concise syntax:

MappingNode(
      "hr"   -> 65,
      "avg"  -> 0.278),
      "blob" -> "nostradamus",
      "bool_true" -> true,
      "bool_false" -> false,
    )

As we know, implicit conversions could be very tricky so I would prefer t place them at org.virtuslab.yaml.syntax to limit accidental import. In addition to import this package, user will have to import import scala.language.implicitConversions or set proper compiler option. Otherwise, he will get compiler warnings about usage of implicit conversion.
I find this proposal as a good balance between convenience and user awareness of what he is doing. What do you think?

[lwronski]

Good point, we should allow to use implicit conversions if the user imported special object from scala-yaml. We will add it in a separate PR.

Please, add at least one test for testing your implicit for nested yaml. For example

Sequence...
         Mapping...
         Mapping...

[kpodsiad]

LGTM, cannot approve because I'm author of the PR 😅