ligature support

[ajshephard]

I'm using the popular Fira Code font and ligatures are not being displayed when using Fortran (Modern). Ligatures appear to be supported with Fortran (Fixed Form). However, with Modern, they only render in the comments, and not in the actual code. So, e.g., operators such as >= are appearing as the separate > and = and not the combined ligature character provided by Fira Code.