Implement Command naming convention

[aslesarenko]

Problem

ErgoTool is supposed to be compiled using native-image, which impose some limitations on architecture. In particular new commands cannot be implemented as dynamically loaded plugins.
Thus in a long term we will end up having a lot of command supporting different application domain.
It will be hard to come up with simple and clear names of the commands.

Solution

All commands can be grouped into namespaces, one namespace for each application domain.
Command names can optionally have namespace prefix followed by command name.

Examples with prefixes: dex:NewToken, dex:SellOrder.

Examples without prefixes: mnemonic, send, etc.

ErgoTool object should become a dispatcher between app-specific command processors (new trait CommandProcessor should be created)
The parser can extract application prefix and create the corresponding processor.
The subsequent parsing and processing should be delegated to the processor.