added support for ToPlainText for pipetables extension

[PaulVrugt]

This PR adds support for ToPlainText for the pipetables extension. Before this, it would simply return all the table html elements, and render plaintext within the table. With this PR, all the table elements are removed from the plaintext output as you would expect as a result of ToPlainText

[xoofx]

Thanks!

[coveralls]

Pull Request Test Coverage Report for Build 2463374125

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 97 of 97 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 93.206%

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Totals
Change from base Build 2457016649:	0.005%
Covered Lines:	25783
Relevant Lines:	27035

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💛 - Coveralls