Feature request: special characters

[burtonator]

This is a nice feature from Froala.. and super easy to implement. It's just a table of math / etc characters that aren't readily available on the keyboard.

It's a good middle path between regular text and full math. You could discuss a lot of mathematical topics without needing a full feature set of mathml

[geektortoise]

Hello,

I made this
https://github.com/geektortoise/Trumbowyg/commit/7aea6048f16f976acc78c0cdd857acc7534c8dc0

But it is really "hacky" : to separate the different categories, I put some null values and I use css on it.

To have a similar window, I think than we need to have a panel that is a mix between the current drop down lists and modal boxes and I don't know how to do it with the current state of Trumbowyg.

If i'm wrong, I'll be happy to adapt my code.

[Alex-D]

I've just merged @geektortoise plugin since it seems good enough in lot of cases.

I keep that in mind for v3 (See #875 for more details about v3)