Allow equations to be parsed

[varkor]

It would be nice if quiver could parse the following:

\[(rf)^a \xto{\eta(rf)^a} r\ell(rf)^a \xto{r\bar\ell_{rf}} r(\ell rf)^b \xto{r(\varepsilon f)^b} rf^b\]

to the corresponding diagram. There may be parsing ambiguities; I would have to think some more about this.