support for parameterizing choice fields by the current entry

[kenegozi]

This opens up contextual select tags, e.g. cascading selects.

e.g to create a cascading set of 'state' and 'city' fields, you can now do that:

 nga.field('state', 'choice').choices(states),
 nga.field('city', 'choice').choices(function(entry) {
     return cities.filter(function(city) { return city.state === entry.values.state;} );
 })

given

 var states = [ { value:'WA', label:'Washington'}, ... ];
 var cities = [ { value:'Bellevue', label: 'Bellevue', state: 'WA'} , ... ];

[fzaninotto]

Great idea!

Although there are a couple more uses of Field.choices, in ChoicesField for instance. Could you also update these ones?

Also, would you mind adding a mention of the new syntax in the README?

[kenegozi]

Good catch. I included ChoicesField, added a few tests/specs (for Choice and Choices), and a mention in the readme.

[jpetitcolas]

👍

[jpetitcolas]

Can you rebase on master, now that tests are fixed?

[fzaninotto]

Superseded by #398