[SEND FEEDBACK] New custom option for "moderate" or "opinionated" completion UIs

Commit 0ff98fd1 introduces the most far-reaching customisation option to date. It provides the means to tweak the aesthetics of completion frameworks, specifically Icomplete, Ido, Ivy, Helm, Selectrum, Sallet, as well as completion styles like Orderless and Flx.

Please consult the README section because the "opinionated" option is UI-dependent. Here are some screenshots with Ivy and then Icomplete.

Ivy standard looks (when the option is nil---the default)

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Screenshot_from_2020-08-17_09-37-47

Ivy "moderate" looks

Screenshot_from_2020-08-17_09-38-04

Screenshot_from_2020-08-17_09-38-06

Ivy "opinionated" looks

Screenshot_from_2020-08-17_09-38-27

Screenshot_from_2020-08-17_09-38-28

Icomplete (and Orderless) standard looks

Screenshot_from_2020-08-17_09-39-34

Screenshot_from_2020-08-17_09-39-36

Icomplete (and Orderless) moderate looks

Screenshot_from_2020-08-17_09-39-48

Screenshot_from_2020-08-17_09-39-50

Icomplete (and Orderless) opinionated looks

Screenshot_from_2020-08-17_09-40-00

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