Add Stalebot exempt tag

[cokia]

Description

I've added a feature that prevents the Stale Bot from being stale with work-in-progress tags, because Stale Bot makes it too fast for us to stack on issues and PR, and Issue and PR, which are being constantly discussed.
The need for this was given at #3683.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run dtslint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success.
• I ran npm run test:cov and my test cases cover all the lines and branches of the added code.
• I ran npm run build and tested dist/web3.min.js in a browser.
• I have tested my code on the live network.
• I have checked the Deploy Preview and it looks correct.
• I have updated the CHANGELOG.md file in the root folder.

[cokia]

@GregTheGreek
please check this PR. Thanks.

[coveralls]

Coverage decreased (-1.001%) to 84.342% when pulling 53928b1 on cokia:1.x into 1fe67a1 on ethereum:1.x.

[GregTheGreek]

Thanks so much!