Add support for ``html_show_sphinx``.

[picnixz]

theme_show_powered_by takes value from html_show_sphinx or html_theme_options['show_powered_by'] when available.

• Fix #172.
• Fix #205.

[AA-Turner]

Would the other way around work? I.e. we change show_powered_by to show_sphinx in layout.html, and then use s_p_b to update the value of show_sphinx?

We should also update the documentation to note that show_sphinx is now preferred.

A

[picnixz]

Would the other way around work? I.e. we change show_powered_by to show_sphinx in layout.html, and then use s_p_b to update the value of show_sphinx?

I think it should work as well. But is there any possibility for themes inheriting alabaster that they would break ? For instance, they could rely on show_powered_by, meaning that we still need to keep some theme_show_powered_by entry in the context (not really an issue but more a redundancy).

We should also update the documentation to note that show_sphinx is now preferred.

Is there an elegant way to deprecate the usage of such option and, if so, what about the deprecation period ?