Date & Search archive meta support

[sybrew]

This is a follow-up of #20, which derailed into adding Post Type Archive support only.

Date and Search archives are nestled deeply within WordPress's Core and have no interface.

This lack of interface, plus the multidimensional and dynamic nature of those archives, make this quite a tricky proposition:

1. Archives spawn from arbitrary search queries, which can be filtered further (by developers and visitors alike).
2. Archives spawn as time moves on.

Since we do not support title & description generation syntax (#140), I see no easy way to implement their most requested features (besides robots-meta).

Yet, we already support robots' metadata configuration for these archives. By default, the date and search archives are marked with noindex.

[sybrew]

This feature might also cover custom 404 titles and descriptions.