Bold greek letters

First of all, thank you for this wonderful package! I had no idea that it was from somebody in CFEL!

I noticed that it is not possible to create bold greek letters at the moment. `\mathbf{ε}` which produces a regular ε, probably because ε is part of the font’s `special_chars`, which will always filter it and redirect to the `NewCMMath-Regular` font.

In usual LaTeX this is circumvented by commands like `\boldsymbol` or `\bm` which do some complicated font replacements (therefore this issue may be connected to #107.

But rather than implementing something arcane like `\boldsymbol`, maybe it would be nice to just make it work out of the box with `\mathbf`: If you are in non-regular font, just ignore the special characters and try to fetch them from the respective font. This would break something like `\mathbf{a \sum b}` but in most cases people would not try to do that anyway, right?

I’m happy to implement it either way.

It turns out that NewCM-Bold contains the bold greek letters, which is enough for me, but not things like bold ∇. No idea where they would be found.