Add support for nullable types, map any as default type : fix #19, fix #31

[abenhamdine]

Add support for nullable types.
For this, refactor mapTableDefinitionToType() : values in tableDefinition are now objects which store several properties, currently : udtName, nullable, tsType.
It allowed me to store nullable property, and it brings some extensibility to handle future properties for columns (eg comments).

I did not put this behind a flag as initially considered, because I don't see any good reason to lost information about nullable type. If a user don't care about checking null types, he would necessarily have disabled --strictNullChecks so this won't disturb him.

Considering this brings support for nullable types, I had to enable strictNullChecks and then I ran into null type error in src/index.ts L37 so I also made some changes here.

Also, as of now, SQL types which don't have defined mapping types will be converted to any type.

NB : this PR is against master, there will be merge conflicts with #32

NB2 : once again, I was unable to make diff test succeed on my PC, although test/artifacts/osm.ts and test/example/osm.ts are identical in VS Code (perhaps a pb related to carriage return encoding ?)

[abenhamdine]

ah CI failed because of 2 lines exceeding 120 characters, I forgot I have disabled this tslint rule on my PC 😞
@xiamx Does this prevent you to review/merge this one ?

[xiamx]

@abenhamdine no problem :) I'll disable lint check on line length.. With type declaration on function arguments, it's very easy to go beyond the line limit