Multiline support?

Hi,

It would be nice to have bash-like multiline support. For example, suppose I'd like to get spot-price-history in Korea region, I type following in one line:

```
aws> ec2 describe-spot-price-history --region=ap-northeast-2 --instance-types c4.large --start-time=$(date +%s) --product-descriptions="Linux/UNIX" --query 'SpotPriceHistory[*].{az:AvailabilityZone, price:SpotPrice}'
```

But if doing same thing on presentation / tech sharing, my audience generally prefers this format (in bash):

```
aws ec2 describe-spot-price-history \
--region=ap-northeast-2 \
--instance-types c4.large \
--start-time=$(date +%s) \
--product-descriptions="Linux/UNIX" \
--query 'SpotPriceHistory[*].{az:AvailabilityZone, price:SpotPrice}'
```

Is it possible to do so in aws-shell?

The issue does not name any files, tests, or entry points. Start by locating aws-shell's command input and parsing flow, then determine how continuation lines should be recognized and submitted. Done means the documented multiline example is accepted and behaves like the equivalent one-line command, with coverage for the new input behavior.

Written by the indexing model from the issue text.