Add FieldDefn subtype

[jdesboeufs]

Hello,

In GDAL (OGR) some types have optional subtypes:

• Integer => Boolean, Integer16
• Real => Float32

The ability to know what subtype it is in a input layer, and to define the proper subtype in an output layer can be very useful (more compact datasets and true booleans in JavaScript or in a database).

I opened a PR as a first step. It will probably need some additional work.
I did not cast integer(boolean) to JS boolean not to introduce breaking changes.

Related to #37

[mmomtchev]

Add at least one unit test please

[jdesboeufs]

I'm so sorry for the delay but I will add unit tests ASAP.

[mmomtchev]

As I have told you a number of times over the last few years, I am currently living in total isolation since my last employers organized a very ugly sexual harassment in order to cover up false rape accusation against me organized by another employer with the help of the French police and the French judiciary. All of them are currently extorting me to stop talking about this affair while people post simultaneously issues on my projects. Additionally, I am getting simultaneous WhatsApp messages offering to sell me cocaine bearing a very similar signature to those that were used for the false rape accusations with the French police except that this was more than a decade ago and back then these were SMS messages.

I don't know if you are in for the dicks, for the drug trade or for the money, but I am on github to make good software - I don't have anything else to do and I have barely enough money to eat - which still allows me to exploit the unique economics of making software - and to use this software to raise awareness about my situation.

As your PR is absolutely in line with my goals, I guarantee you that nothing will stand in its way, besides the fact that it lacks a unit test.