Create json schema for parser results

[geokala]

A JSON schema should be created for the parser results (and probably used in the tests).

See json-schema.org

[geokala]

Current schema is roughly (check code to confirm some parts):
feature:
header: []
tags: []
background: <something- same format as other elements?>,
leading_space_and_comments: []
trailing_space_and_comments: []
elements: [
{
'type': <scenario|scenario outline>,
'description': ,
'tags': [], ** optional for scenario/outline; not valid for background
'leading_comments_and_space': [],
'examples': [
{
'leading_comments_and_space': [],
'description': (if any),
'columns': {
'<row 1 from column 1 from data>': [rest,of,row,1],
'<row 1 from column 2 from data>': [rest,of,row,2],
},
'trailing_whitespace': [],
},
...
], ** required for scenario outline; not valid otherwise
'steps': [
{
'leading_comments_and_space': [],
'type': <given|when|then|and|but>,
'text': <text of step without type (Given) I have a testing library>,
'multiline_arg':

,
'trailing_whitespace': [],
},
...
],
}
] ** last element must be a scenario or scenario outline

[geokala]

PR in place: #36