GraphQL plugin

What you think about implement GraphQL plugin to bitshares-core?

GraphQL allow make requests for data you need.
For example, if I need get orders and balances of account.
I call:
```
get_full_accounts(['scientistnik'], false)
```
I get:
```
[ 'account',
'statistics',
'registrar_name',
'referrer_name',
'lifetime_referrer_name',
'votes',
'cashback_balance',
'balances',
'vesting_balances',
'limit_orders',
'call_orders',
'settle_orders',
'proposals',
'assets',
'withdraws',
'htlcs']
```
But, I need only `balances` and `limit_orders`.
In GraphQL:
```
{
account(name='scientistnik') {
balances {
asset_id
amount
}
limit_orders {
for_sale
sell_price
}
}
}
```

No source files, tests, or entry points are named. Start by reviewing the bitshares-core plugin architecture and the GraphQL account query examples in the issue; done means a defined plugin can return only requested account fields such as balances and limit_orders.

Written by the indexing model from the issue text.