Delay item class object creation

[Gallaecio]

Fixes #14

[Gallaecio]

Reached a point where I think we need to make changes in itemadapter first. I think we need to be able to change:

ItemAdapter(self.item).get_field_meta(field_name)

So that ItemAdapter works the same getting an item class instead of an item object.

[ejulio]

I was thinking a bit about it and it seems to be a good solution, even though it might not solve all the cases.

It'll allow users to not set all fields as optional, but in case you build the item from multiple requests (or nested loaders), you'll still need to set these fields as optional.
In the nested loaders case, it can be a problem, as I'm setting something that shouldn't be optional as optional.
In the multiple requests one, it can be seen as proper modeling, I'm create an intermediary item and completing it later. So in this case, for a proper "model", the user should have 2 classes, one for the intermediary case and a final one for the full item.

[codecov]

Codecov Report

Merging #20 (a09e0d6) into master (811985a) will increase coverage by 0.39%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master       #20      +/-   ##
===========================================
+ Coverage   99.60%   100.00%   +0.39%     
===========================================
  Files           4         4              
  Lines         254       269      +15     
===========================================
+ Hits          253       269      +16     
+ Misses          1         0       -1     

Impacted Files	Coverage Δ
itemloaders/__init__.py	100.00% <100.00%> (+0.64%)	⬆️

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[Gallaecio]

I’m not sure if this should go or stay; “shares the item” is not technically accurate any more, but maybe we should simply reword it so that it indicates that load_item() returns the same object regardless of whether it is called on the original item loader or in a nested item loader.