support for line symbol

Add support for ```line(AB)``` to generate double-ended arrow above AB. Expressions like ```bar(AB)``` and ```vec(AB)``` are supported so seems reasonable that ```line(AB)``` is too.

**This breaks backwards compatibility**

Before this change

    > am('line').toString()
    '<math title="line"><mi>l</mi><mo>&isin;</mo><mi>e</mi></math>'

After this change

    > am('line').toString()
    '<math title="line"><mover><mo></mo><mo>&harr;</mo></mover></math>'
    
    > am('l in e').toString()
    '<math title="l in e"><mi>l</mi><mo>&isin;</mo><mi>e</mi></math>'