Warnings should be printed in separate mode

[buchslava]

We should not to see warnings by default. Only errors. We should use extra parameter during start of program to show warnings. For example,

validate-ddf <folder> -w - with warnings
validate-ddf <folder> - without warnings

[buchslava]

I think this idea will be very useful for next case:

[{
    "type": "Incorrect JSON field",
    "path": "/home/slava/q3/ddf--concepts.csv",
    "data": {
        "column": "drill_up",
        "line": 14,
        "value": "['geographic_regions', 'income_groups', 'landlocked', 'g77_and_oecd_countries', 'geographic_regions_in_4_colors', 'main_religion_2008']"
    },
    "suggestions": ["[\"geographic_regions\",\"income_groups\",\"landlocked\",\"g77_and_oecd_countries\",\"geographic_regions_in_4_colors\",\"main_religion_2008\"]"],
    "warning": true
}, {
    "type": "Incorrect JSON field",
    "path": "/home/slava/q3/ddf--entities--geo--country.csv",
    "data": {
        "column": "arb5",
        "line": 55,
        "value": "Congo [DRC]"
    },
    "warning": true
}]

I mean next: "value": "Congo [DRC]"

this value looks like wrong JSON (from robot side), but we know that it's not true

[buchslava]

duplicated with #124