Extend statement initiializer by the command name

I realized that it makes sense to initialize the command using a name. It's ok for the WIP JSON deserializer, since the parser wil see the command name before it sees the arguments _always_. Other formats should take care to have the command name before the argument list, which should be the case anyway.

This has two concrete benefits:
 * The parsers don't need to know anything about `command/command.h` now.
 * Statement construction get's even easier.