Add way to manually link eosio.code permission

[nsjames]

Currently there is no easy way to add the eosio.code permission easily from dune. It requires the developer to dig into cleos and do a moderately hard task (getting the key, setting up the json, formatting the command, etc)

This is a fairly common task for devs.

It would be nice if:

• The code permission was added on deployment as a flag
• The code permission was added during account creation
• Adding a code permission is its own command (my personal preference)

[nsjames]

It looks like it already does automatic addition of that perm when deploying code (thanks @larryk85):

https://github.com/AntelopeIO/DUNE/blob/206a07aae938658ea69cc362a36c7669a1bbf0c7/src/dune/dune.py#L687