output flag not respected for "tx <module> <command>"

[yaruwangway]

Summary

Problem Definition

Proposal

• the query command has the option -o json
• the tx bank send command does not have the option -o json
  It is helpful to have json output for tx bank send as well, it is easy to parse.

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[alexanderbez]

Thanks @yaruwangway!

Note, this is not particular to x/bank, but to all tx commands.

1. Add the output flag to AddTxFlagsToCmd
2. Make sure we populate the context in readTxCommandFlags