Enable insertion of symbols without math-mode delimiters

[skye-repos]

Julia code can make extensive use of unicode math symbols. When trying to use cdlatex-mode, the $$ was inserted. This is a sane default for when writing latex or in org-mode, but in julia, those delimiters create issues with julia-mode's substitions

A small modification to logic of the =cdlatex-ensure-math= function allows for the use of arbitrary strings as delimiters or even no parens/dollars. We could maybe alter it to take in a choice from an existing list of accepted values if need be.

Best, Skye