Add a plugins keyword argument for doctests (and makedocs)

[goerz]

From JuliaDocs/DocumenterCitations.jl#34, downstream from CliMA/ClimaAtmos.jl#2068

At least in principle, it would be conceivable that a Documenter plugin could do something that would affect doctests. It would be good if there was some way to pass plugins to the doctest function.

As a matter of fact (since there's a breaking 1.0 release coming), it might be worth reconsidering how Documenter handles plugins in general. The way plugins are passed as positional arguments is undocumented, see #2113. Maybe it would be better to have a plugins keyword argument for both makedocs and doctests?

[mortenpi]

Yeah, I would be okay with changing the positional argument to a keyword in makedocs. The positional arguments are a bit out of place considering that everything else is done via keywords. And yes, I think doctest should also take that keyword.

[goerz]

Great, I'll take a stab at it maybe this weekend

[goerz]

Done, see #2249