Add stdout output

[tlvince]

Unfortunately GitHub Advanced Security (for the SARIF feature) is unavailable for private repos on github.com. As an alternative, Slither's markdown report can be used.

This sets outputs.stdout (which is required to be a single-line string) and adds example usage to the readme. See tlvince/hardhat-project#1 (comment) to see how it looks.

[CLAassistant]

All committers have signed the CLA.

[stephenctw]

Nice work! I faced the same issue that Github Advanced Security is not available for private repos. The way I worked around it is to add another step to parse the SARIF report into github annotation.
https://github.com/marketplace/actions/process-sarif-file

[abcfy2]

Really need this feature, we need to use the markdown report. Please merge this PR. Thanks.

[elopez]

Thanks for the PR @tlvince! I've merged dev to update the branch, updated the README to match, and modified the escaping code to use the new multiline strings feature on Actions output. I think we can merge this now, unless you have any objections to the changes I did.

[tlvince]

lgtm 👍🏻