Quoted Printable field support

[JamesMenzies]

Adding support for Quoted Printable fields

Related to Issue #448

• GetContentLines string parsers to support line continuation when Encoding is quoted printable AND line ends with =
• handle fields with encoding quoted printable string fields.
• support utf8, utf16, and SJIS encoding in quoted printable encoding.

I am far from convinced that this is a good solution. There are a number of places where this pull seems inelegant.

1. The implementation I have made in of GetContentLines that tests for the encoding by a string match is ugly. I would prefer to, get the field name, read the properties, and then use that information while parsing the value but that is structural change.
2. I think I've made an untidy thing in the DataTypes/QuotedPrintableString.cs I think I should be wrapping the value in a strongly typed object. The way I am using it as something like a Type flag feels more like a hack.
3. encoding name matching added to GetEncoderFor isn't pretty.
4. The trick to support SJIS encoding is secret magic. I think you just have to live with in .netstandard it but in is a bear trap that will eat someone.