Feature request: Add support for setting bold italic math font

[muzimuzhi]

When mathspec is used to set font of math alphabets, users may also expect to use a bold italic math font version. Normally this work is done by \bm from bm package, but with mathspec \bm does not work anymore. Hence it might be the responsibility of mathspec to provide a bold italic math font version command (or patch \bm if bm is loaded).

The solution to first suggection can be as simple as

diff --git a/mathspec.sty b/mathspec.sty
index e63a27d..32a47f6 100644
--- a/mathspec.sty
+++ b/mathspec.sty
@@ -940,6 +940,7 @@
   \SetMathAlphabet{\mathrm}{bold}{\eu@enc}{\eu@mathrm}{bx}{n}
   \DeclareMathAlphabet{\mathit}{\eu@enc}{\eu@mathrm}{m}{it}
   \DeclareMathAlphabet{\mathbf}{\eu@enc}{\eu@mathrm}{bx}{n}
+  \DeclareMathAlphabet{\mathbfit}{\eu@enc}{\eu@mathrm}{bx}{it}
   \SetMathAlphabet{\mathit}{bold}{\eu@enc}{\eu@mathrm}{bx}{it}
   \DeclareSymbolFont{Operators:m:n}{\eu@enc}{\eu@mathrm}{m}{n}
   \def\operator@font{\expandafter\mathgroup\csname symOperators:m:n\endcsname}}

Maybe a new pacakge option controlling whether to provide \mathbfit is needed.

---

I know that unicode-math is more recommended, but since mathspec still exists, feature request like this one may still have its meaning.