Support math.style.exponents

Support math.style.exponents

this option to print.xtable supports the values TRUE or $$ which makes
1e5 like $1\times 10^{5}$ or a value of "ensuremath" which uses
\ensuremath instead of $$ or a value of "UTF-8" or "UTF8", which uses
UTF-8 to approximate the LaTeX typesetting (e.g., 1×10⁵)