Allow user to extend set of supported types for YAML parsing/writing.

It would be nice to allow users to provide YAML files that can directly be parsed into some structs. E.g., it would be nice to parse
```
My Complex: 1.0 + 3i
My Int Pairs: [ P(1,2), P(0,1)]
```
directly into a `std::complex` and a `std::vector>` respectively. Even more interesting/useful would be to parse straight into enums, so that one can do
```
auto plist = parse_yaml_file("blah.yaml");
MyEnum value = plist.get("The Enum");
...
```
without having to tediously do string->enum conversion every time.

To do this, we should put the `is_type` and `str2type` functions into a separate file. Then, somehow, allow the user to provide its own specializations (along with `operator<<` for writing, if writing/print is needed). One way could be to have a "database" of these functions, and let the user register its own functions at any point before calling the YAML parse/write routines.