Added support for total count on relay connections

[CaselIT]

This PR adds support for returning the length of a ConnectionField under the argument totalCount
Example query:
{ allArticles { totalCount } }
will return
{ "allArticles": { "totalCount": <number of articles> } }

This option is added by default, but can be disabled by adding total_count = False to the Meta of a node line as in the example below:

class Role(SQLAlchemyObjectType):
    class Meta:
        model = RoleModel
        interfaces = (relay.Node, )
        # Disable the total count on this connection
        total_count = False

I've added the tests for this, but I do not know where should I add the documentation. What's your advice @syrusakbary
Closes #58

[coveralls]

Coverage increased (+0.2%) to 91.186% when pulling 99ec6b4 on CaselIT:total_count into 4827ce2 on graphql-python:master.

[CaselIT]

@syrusakbary have you had a chance to review this pr?

Thanks

[CaselIT]

@syrusakbary sorry to bother you again. It's been more than a month without any feedback and I just wanted if you are still supporting this library or if I should consider creating another package
Thank you

[microidea]

@CaselIT I've already merged your code into my project 😂, can't wait that long.

[exit99]

Can we merge this in? Highly needed

[MichaelWashburnJr]

Why the hell was this never merged in?

[exit99]

@syrusakbary I'm assuming this was not merged in, as you can just override the classes yourself?
Or is there another reason?

[CaselIT]

@Kazanz you can override the class, but for it to work everywhere you need to patch the import in the library before creating any types with something along the lines of

from graphene_sqlalchemy import types
types.Connection = ConnectionWithCount

which is clearly not the best way to achieve this.

I'm not really sure why it has not been merged.

[nikordaris]

I've submitted a more general solution that will enable you to define your own Connection with count #120.

[CaselIT]

@nikordaris Nice. I'll close this one in favor of your implementation. Hopefully it will be merged..