add MultiExec.add(*Commands)

[samuelkees]

Adds add to MultiExec to add Commands.

use:

from reddish.trio import MultiExec, Command

tx = MultiExec(
    Command('ECHO {}', 'foo'),
    Command('ECHO {}', 'bar')
)
# ... same code hire 

tx.add(
    Command('ECHO {}', 'baz'),
    Command('ECHO {}', 'qux')
)
foo, bar, baz, qux = await redis.execute(tx)