skip escaping some unicode math symbols

[arades79]

add some math symbols to skip glib escape for -calc-command

Fixes #108

If needed, I could put the extra escapes behind an argument like -calc-command-skip-escapes. Not sure of all the -calc-command use-cases and how this could affect them.

[svenstaro]

Cool though please also add a comment explaining why we do that.

[arades79]

Done

[svenstaro]

Cool, thanks!