Support both MathJax and KaTeX + allow configuration

[mortenpi]

Add the mathengine argument to HTML which allows the math rendering engine to be specified and support both MathJax and KaTeX (the latter is the default).

Furthermore, you can also pass custom configuration to the rendering engine. E.g. to add global LaTeX command definitions, you can set mathengine to:

mathengine = Documenter.MathJax(Dict(:TeX => Dict(
    :equationNumbers => Dict(:autoNumber => "AMS"),
    :Macros => Dict(
        :ket => ["|#1\\rangle", 1],
        :bra => ["\\langle#1|", 1],
    ),
)))

Builds on top of #1092. Close #981 .