Add support for "name:type" style Headers

[cowtowncoder]

Currently use of Header row (and manual construction of CsvSchema) assume that headers only specify name of column position. But there exists usage, wherein header line contain type mappings too:

name:string,enabled:boolean, ...

It would be great to support this.

There is an immediate problem with "type" definition: as things are, only a very limited set of types is recognized for CsvColumn. But we could divide implementation into two main parts:

1. Support mapping to existing small set of types (probably allowing case-insensitive type definition)
2. Support configurability of extension types, by providing converters from String value into target types.

For (2) there is also additional step needed: whereas we could expose JsonToken values for:

1. Strings
2. Numbers
3. Booleans
4. nulls

everything else would need to be exposed as JsonToken.VALUE_EMBEDDED_OBJECT.
But if so, any and all types could be supported -- as long as jackson-databind deserializers (JsonDeserializer impelmentations) consider possibility of embedded type.

Doing above is not trivial, but would be a very powerful mechanism for customized binding from CSV data. Some amount of duplication would likely result (for Converters) but that seems acceptable.