Add a time expression provider to allow the user to select the predefined ones.

[luqp]

Fixes #816

Add support for the Humanize() method to work with other time expressions so that the user can select from the predefined.

@mehdikhalili, @onovotny, @hazzik, @mexx Could you please review this? If the approach is good I can continue updating the documentation and apply your suggestions.
Thank you.

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