Use CSVW default dialect

[RickMoynihan]

See also #342

We should:

1. assume the default dialect if one is not provided.
2. validate that any dialects provided in the metadata conform to the dialect schema (from the CSVW spec).
3. validate that they are also in our supported subset of dialects (happy to just go with the default for now), in which case we should fail on anything else.... e.g. failing on anything that isn't our default delimiters and explicitly UTF-8 would be fine.

We should probably split this up into several issues.

The default dialect in the spec is:

{
  "encoding": "utf-8",
  "lineTerminators": ["\r\n", "\n"],
  "quoteChar": "\"",
  "doubleQuote": true,
  "skipRows": 0,
  "commentPrefix": "#",
  "header": true,
  "headerRowCount": 1,
  "delimiter": ",",
  "skipColumns": 0,
  "skipBlankRows": false,
  "skipInitialSpace": false,
  "trim": false
}