Allow to specify the config to include all the mempool transactions in the newly generated block

[LogvinovLeon]

I'm running:

• Parity version: latest master
• Operating system: MacOS
• And installed: from source

---

Currently there is an option to force parity to generate a fake block with some pending transactions even if it's not mining.
--force-sealing
It grabs some transactions from the mempool and creates a fake block out of them.
It's usefull, cause later you can get pending logs by using eth_getLogs and you'll get all the logs from this fake block.
I want to have a config to make a block out of all the transactions currently in the mempool.
This way - I'll be able to see - what each of them does.

My use-case:
I have a dApp and I want my UI to update faster, then 15s after a user action. To update the UI I need to know, what will happen on the blockchain. Now I have a way to know what will happen in the next blok, but during the ICO's and other periods of network being full and mempool being much bigger then 1 block this is not very usefull, cause I might see an update after a couple of hours.
Yes, I can just fetch the transactions from the mempool, but I'll need to have a local VM running in order to get any usefull info from them. I feel like this use-case is generic enough to be supported.