Add option to specify which events to listen for in listen command

[romac]

Closes: #550

Description

USAGE:
    hermes listen <OPTIONS>

DESCRIPTION:
    Listen to and display IBC events emitted by a chain

POSITIONAL ARGUMENTS:
    chain_id                  Identifier of the chain to listen for events from

FLAGS:
    -e, --event EVENT         Add an event type to listen for, can be repeated. Listen for all events by default (available: Tx, NewBlock)

---

Listen for default events:

$ hermes listen ibc-0
[info ] Listening for events `NewBlock, Tx` on 'ibc-0'...

Listen for NewBlock event only:

$ hermes listen ibc-0 -e NewBlock
[info ] Listening for events `NewBlock` on 'ibc-0'...

Listen for NewBlock and Tx events:

$ hermes listen ibc-0 -e NewBlock -e Tx
[info ] Listening for events `NewBlock, Tx` on 'ibc-0'...

---

For contributor use:

• Updated the Unreleased section of CHANGELOG.md with the issue.
• If applicable: Unit tests written, added test to CI.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments.
• Re-reviewed Files changed in the Github PR explorer.

[ancazamfir]

Looks great, thanks Romain!