[Improvement] Parachain should generate spec

[crystalin]

The parachains should generate the specs, like the relay does, in order to provide custom specs changes (like on the relay) in the config.
This would allow also to have the raw specs available (as a file) for extra node to be booted on that same network.

[joelamouche]

tackles #151

[joelamouche]

Working on this.
Does export-genesis-state still support --parachain-id= or should we remove it as well

[bkchr]

It doesn't support this anymore.

[brenzi]

this would also allow to change genesis values for parachains, which would be very handy