Support for multi-colunm align

[Dekel Tsur]

The amsmath align environment can have more than 2 columns:
\begin{align}
1 & 2 & 3 & 4\\
5 & 6 & 7 & 8
\end{align}

Is it hard to add support for this ?

Note that we don't know the number of columns in advance, so
mathed_parse_lines() should increase the number of columns in the inset when
it reads the first line.