Add option to prefix original attribute-pairs

[kimor79]

This adds an option to prefix the existing record.

E.g., input like: {"message":"a:A b:B c:C","source":"xyz"}

with config as:

type parser
format ltsv
reserve_data yes
reserve_data_prefix @

becomes

{
  "@message":"a:A b:B c:C",
  "@source":"xyz",
  "a":"A",
  "b":"B",
  "c":"C"
}

[tagomoris]

Could you rebase on master HEAD, and add tests for your patch?