Ability to skip columns

[roll]

• fixes Ability to skip columns #248

[roll]

@akariv
@amercader
This adds an ability to skip arbitrary columns for both tabulator and goodtables (see the docs in the changeset)

For example for goodtagles:

data

other1,sample_id,fermentation_id,strain_id,time,other2
,s1,f1,s1,10,some
some,s1,f1,s1,bad,some

metadata

{
  "fields": [
    {
      "type": "string",
      "name": "sample_id",
      "format": "default"
    },
    {
      "type": "string",
      "name": "fermentation_id",
      "format": "default"
    },
    {
      "type": "string",
      "name": "strain_id",
      "format": "default"
    },
    {
      "type": "integer",
      "name": "time",
      "format": "default"
    }
  ]
}

report

{u'error-count': 1,
 u'preset': u'table',
 u'table-count': 1,
 u'tables': [{u'encoding': 'utf-8',
              u'error-count': 1,
              u'errors': [{u'code': u'type-or-format-error',
                           u'column-number': 4,
                           u'message': u'The value "bad" in row 3 and column 4 is not type "integer" and format "default"',
                           u'message-data': {u'field_format': u'"default"',
                                             u'field_type': u'"integer"',
                                             u'value': u'"bad"'},
                           u'row-number': 3}],
              u'format': 'csv',
              u'headers': [u'sample_id',
                           u'fermentation_id',
                           u'strain_id',
                           u'time'],
              u'row-count': 3,
              u'schema': u'table-schema',
              u'scheme': u'file',
              u'source': 'tmp/sample_info.csv',
              u'time': 0.008,
              u'valid': False}],
 u'time': 0.126,
 u'valid': False,
 u'warnings': []}

[amercader]

This looks good to me @roll. Just some nitpicking about the docstrings

[amercader]

Perhaps "When passed, ignores all columns included in the given list" is more clear

[amercader]

Perhaps "When passed, ignores all columns that are NOT included in the given list" is more clear

[amercader]

Same as above

[roll]

@amercader
Thanks!