parameters comment instead of tag

• What: support writing #parameters as a comment at the top of a cell as an alternative way to tag
• Why: it's annoying to use the tags mechanism, especially if it's just for papermill (related: #192)

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Paraphrasing https://papermill.readthedocs.io/en/latest/usage-parameterize.html#how-parameters-work (which I may have written :P):

1. if multiple parameters tags are found, injection only occurs after the first one (related: #328)
2. if none are found, injection is at the start of the notebook

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proposed here: insert 2. if none are found, injection occurs after the first cell that starts with #parameters (possibly also variants such as """ PaRaMeTeRs""")

This will not break backward-compatibility.

Happy to implement this if there's support for it.