Dialect hash. Support key symbol.

[kwent]

Hi,

Dialect hash should support ruby key symbol.

dialect = {
    header: true,
    delimiter:  ","
}
validator = Csvlint::Validator.new( "http://example.org/data.csv", dialect )

Right now if we are doing that it's all ignored and it's confusing (yes i did the mistake to put key symbol instead key string).

Regards