feat(scripts): load chain env variables on demand

[nseguias]

Description and Motivation

This PR enhances the dev experience by a lot, by loading the desired environmental variables into the terminal for an specific chain.

Then, a generic use of queries or transactions can be performed, making commands composable and also saving time.

The exported variables are:

CHAIN_ID, DENOM, BINARY, RPC and the TXFLAG for transactions.

Related Issues

Closes #241

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