Extract option parsing

[lucapette]

Built on top of #43 and for that reason I'm opening the PR against that branch. So that understanding what I've done in the context of this one is easier to read.

There are a few unrelated changes that create some noise (renaming, some gardening) so let me explain what's the core of this change.

Before this change, fakedata would do the following:

• Take all the args from the CLI
• return an array of columns
• For each row:
  • parse the options of each arg
  • generate a value based on the constraint

This change affects the algorithm so that we do parsing of the options only once. After the change:

• Take all the args from the CLI
• Parse options for each arg
• Return a column with its own Generator embedded (that's the key change)
• For each row:
  • generate a value using the "column generator"

It's a lot of changes but I believe it leads to better design. The two pass-algorithm removes the need for caches (like the fileCache we introduced in the file generator) and opens the door to any optimization we'd like to do in the future (in case it's needed).

I introduced a private factory too, as a consequence of this conversation which removed the init func we have.

[lucapette]

This is the main reason why I'm happier with this design that the current master :)

[lucapette]

The diff doesn't help here. But getGenerator (which I'm thinking of renaming into newGenerator but always not sure about naming) is the core of the change.

We fetch the generator from the factory and the switch does all the work for the "generators with customOptions. I think over time we can make this even "more obvious" (using interfaces and structs) but it's good enough for me now that we have only 3/4 generators with "custom needs"