Accept custom dialects

[RomainTT]

First of all, thanks for this package it’s great!

My problem is that the validator only accepts “classic” CSV with a header, a comma as separator, a newline with \n, etc.

It would be really nice to manage other formats, and even nicer to make these formats part of the schema.

To manage formats, here are some ideas:

• Use the Dialects of the native csv library.
• In this line which reads the CSV file, give a custom dialect to DictReader.
• The custom dialect could either be given as an argument of ValidateCsv.__init__, or could be generated from the schema itself. In that last case, a new method generate_dialect could parse the schema and build the dialect.

Example of schema with maximum complexity for the dialect:

{
  "name": null,
  "description": null,
  "filename": {
    "regex": null
  },
  "dialect": {
    "delimiter": "\t",
    "skipinitialspace": true,
    "lineterminator": "\n",
    "quoting": "QUOTE_NONE",
    "quotechar": "'",
    "escapechar": "\\",
    "doublequote": false
  }
  "columns": []
}

I’ll try to do a PR if I can spare some time.