Accept multiple input files

[dosmanak]

/feature The output file argument should be prepended with -o |--output and the tool should accept as many intput files as defined. The usage will be e.g.

yaml2json [-i <indent>] [-a] [-f] [<yaml_file>, <yaml_file>...] [-o <json_file>]

The output will be json stream

For the json2yaml the output will be yaml multidocument