support column title string set by user

[ynishi]

• support explicit title setting.
• now, inherit papaparse json support http://papaparse.com/docs#json-to-csv
• add this style

var csv = convert(
  header: {
    "Column1": "ColumnTitle1",
    "Column2": "ColumnTitle2"
  },
  data: [
	{
		"Column1": "foo",
		"Column2": "bar"
	},
	{
		"Column1": "abc",
		"Column2": "def"
	}]);

[ynishi]

• Props: header, dataJson

Props:{
  ...
  header: {
    "Column1": "ColumnTitle1",
    "Column2": "ColumnTitle2"
  },
  data: [
	{
		"Column1": "foo",
		"Column2": "bar"
	},
	{
		"Column1": "abc",
		"Column2": "def"
	}],
  ...
};