Make sure 'yamler' supports line numbers

[Sergei Lebedev]

We want to have a line number for every possible line parsed by yamler:

# A random YAML sample.
receipt:     Oz-Ware Purchase Invoice
date:        2007-08-06
customer:
    given:   Dorothy
    family:  Gale

So for example if family is mistyped we could emit a sane error message wih line #5. WARNING: it looks like libyaml trims comments before any further processing, is there a way to change that?