Change \setmathfont syntax to match fontspec v2.4

[jpintar]

Since v2.4, the preferred fontspec syntax for \setmainfont has the optional font features argument following the mandatory font name argument. Here, \setmathfont is changed to follow the same syntax, and the documentation is updated accordingly.

[wspr]

Thanks for spending the time to do this :)

As you might be able to guess, I had intended to fix this up in unicode-math shortly after making the change in fontspec, but life got away with me. The Github version of fontspec uses a bit more of a sophisticated way of parsing the options, but the Github version is also broken in other areas :) I'll make sure this is unified properly between the two packages as soon as possible…