Allow to override E elements

[qcoumes]

I need to customize the behavior of some elements inside ElasticsearchQueryBuilder. Sadly it is not currently possible to just change these element without overriding every method to use my custom element.

Could you consider making these elements an attribute of the class so that we can override them more easily, i.e. something like :

class ElasticsearchQueryBuilder(LuceneTreeVisitorV2):
    
    E_MUST = EMust
    E_SHOULD = Eshould
    [...]

I can make a PR if you agree with the idea.

[alexgarel]

Sounds good to me @qcoumes

[alexgarel]

Resolved by @qcoumes PR #75 !